Kurt Gödel: Essays for his Centennial (Lecture Notes in Logic, Volume 33)

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CUUS1004-Feferman

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Kurt Gödel Essays for His Centennial

Kurt Gödel (1906–1978) did groundbreaking work that transformed logic and other important aspects of our understanding of mathematics, especially his proof of the incompleteness of formalized arithmetic. This book on different aspects of his work and on subjects in which his ideas have contemporary resonance includes papers from a May 2006 symposium celebrating Gödel’s centennial as well as papers from a 2004 symposium. Proof theory, set theory, philosophy of mathematics, and the editing of Gödel’s writings are among the topics covered. Several chapters discuss his intellectual development and his relation to predecessors and contemporaries such as Hilbert, Carnap, and Herbrand. Others consider his views on justification in set theory in light of more recent work and contemporary echoes of his incompleteness theorems and the concept of constructible set.

Solomon Feferman was a professor of mathematics and philosophy at Stanford University from 1956 until his retirement in 2004. He is a Fellow of the American Academy of Arts and Sciences, was President of the Association for Symbolic Logic in 1980–1982, and was the recipient of the Rolf Schock Prize for Logic and Philosophy in 2003. Feferman was editor-in-chief of the Collected Works of Kurt Gödel (1986–2003). Charles Parsons is Edgar Pierce Professor of Philosophy, Emeritus, at Harvard University. He retired in 2005 but has subsequently been a visiting professor at UCLA and the University of Chicago. Parsons is the former longtime editor of the Journal of Philosophy and a former editor of Bulletin of Symbolic Logic. He is the author of Mathematical Thought and Its Objects and a co-editor of Volumes III–V of Kurt Gödel’s Collected Works. Stephen G. Simpson is a mathematics professor at the Pennsylvania State University. He has lectured and published widely in mathematical logic and the foundations of mathematics. Simpson is the developer of the foundational program known as Reverse Mathematics and the author of Subsystems of Second Order Arithmetic, second edition.

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LECTURE NOTES IN LOGIC

A Publication of The Association for Symbolic Logic This series serves researchers, teachers, and students in the field of symbolic logic, broadly interpreted. The aim of the series is to bring publications to the logic community with the least possible delay and to provide rapid dissemination of the latest research. Scientific quality is the overriding criterion by which submissions are evaluated. Editorial Board Anand Pillay, Managing Editor Department of Pure Mathematics, School of Mathematics, University of Leeds Jeremy Avigad Department of Philosophy, Carnegie Mellon University Lance Fortnow Department of Computer Science, University of Chicago Vladimir Kanovei Institute for Information Transmission Problems, Moscow Shaughan Lavine Department of Philosophy, The University of Arizona Steffen Lempp Department of Mathematics, University of Wisconsin

More information can be found at http://www.aslonline.org/books-lnl.html.

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Kurt Gödel Essays for His Centennial Edited by

SOLOMON FEFERMAN Stanford University

CHARLES PARSONS Harvard University, Emeritus

STEVEN G. SIMPSON Pennsylvania State University

association for symbolic logic

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CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Dubai, Tokyo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521115148 © Association of Symbolic Logic 2010 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2010 ISBN-13

978-0-511-74456-3

eBook (EBL)

ISBN-13

978-0-521-11514-8

Hardback

Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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GENERAL Solomon Feferman ¨ The Godel editorial project: A synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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John W. Dawson, Jr., and Cheryl A. Dawson ¨ Future tasks for Godel scholars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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PROOF THEORY Jeremy Avigad ¨ Godel and the metamathematical tradition . . . . . . . . . . . . . . . . . . . . . . . . .

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Wilfried Sieg ¨ Only two letters: The correspondence between Herbrand and Godel

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W. W. Tait ¨ Godel’s reformulation of Gentzen’s first consistency proof for arithmetic: The no-counterexample interpretation . . . . . . . . . . . . . . . . . .

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W. W. Tait ¨ Godel on intuition and on Hilbert’s finitism . . . . . . . . . . . . . . . . . . . . . . . .

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Stephen G. Simpson ¨ The Godel hierarchy and reverse mathematics . . . . . . . . . . . . . . . . . . . . . . 109 John P. Burgess On the outside looking in: A caution about conservativeness . . . . . . . . 128

SET THEORY Akihiro Kanamori ¨ Godel and set theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 vii

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Sy-David Friedman ¨ Generalisations of Godel’s universe of constructible sets . . . . . . . . . . . . 181 Peter Koellner On the question of absolute undecidability . . . . . . . . . . . . . . . . . . . . . . . . . 189 PHILOSOPHY OF MATHEMATICS Martin Davis ¨ What did Godel believe and when did he believe it? . . . . . . . . . . . . . . . . . 229 Warren Goldfarb ¨ On Godel’s way in: The influence of Rudolf Carnap . . . . . . . . . . . . . . . . 242 Steve Awodey and A. W. Carus ¨ Godel and Carnap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 Mark van Atten and Juliette Kennedy ¨ . . . . . . . . . . . . . . . . . . . 275 On the philosophical development of Kurt Godel Charles Parsons ¨ Platonism and mathematical intuition in Kurt Godel’s thought . . . . . . 326 Donald A. Martin ¨ Godel’s conceptual realism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

INTRODUCTION ¨ The year 2006 marked the centennial of Kurt Godel, who was born on ¨ 28 April 1906. The importance of Godel’s work for nearly all areas of logic and foundations of mathematics hardly needs to be explained to our readers. The year 2006 saw several centennial observances. In particular, the program committee for the 2006 Association for Symbolic Logic annual meeting, which took place on 17–21 May at the Université du Québec a` Montréal, commissioned a subcommittee to arrange a portion of the program that ¨ would commemorate the Godel centennial. The subcommittee arranged three one-hour lectures, by Jeremy Avigad, Sy-David Friedman, and Akihiro ¨ Kanamori. It also arranged a two-hour special session on Godel’s philosophy of mathematics, with lectures by Steve Awodey, John Burgess, and William Tait.1 All of the lectures have led to papers in this volume.2 The volume con¨ tains one other new paper, “The Godel hierarchy and reverse mathematics,” by Stephen G. Simpson. Other papers included in the volume are reprinted, in all but one case from The Bulletin of Symbolic Logic. We have included the papers presented at the 2004 ASL annual meeting at Carnegie-Mellon University, in a ¨ special session organized by the editors of Godel’s Collected Works, by Martin Davis, John W. Dawson, Jr., Cheryl A. Dawson, Solomon Feferman, Warren Goldfarb, Donald A. Martin, Wilfried Sieg, and William Tait. These appeared in the June 2005 Bulletin. Also reprinted are papers by Mark van Atten and Juliette Kennedy and by Charles Parsons that appeared in earlier issues of the Bulletin, as well as a paper by Peter Koellner that appeared in Philosophia Mathematica. The subcommittee’s planning of the program and the editors’ work on ¨ this volume have aimed at a collection that would cover Godel’s work in the fields represented by the Association for Symbolic Logic, thus mathematical 1 An attempt to arrange an evening session for a more general audience, consisting either of personal reminiscences of Gödel or of a general exposition of some of his work, was unsuccessful. The subcommittee consisted of Solomon Feferman, Warren Goldfarb, Charles Parsons (Chair), Dana Scott, and Stephen G. Simpson. 2 Kanamori’s paper is reprinted from The Bulletin of Symbolic Logic, vol. 13 (2007), pp. 153– 188. Awodey’s lecture reflected joint work with A. W. Carus, who is a co-author of the resulting paper.

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logic, especially proof theory and set theory, and philosophy of mathematics. ¨ For this reason other aspects of Godel’s work were omitted, in particular his contribution to general relativity, his philosophical reflection on relativity theory, and other parts of his philosophical thought. We have not aimed at comprehensiveness even in the fields within our purview. Nonetheless we hope that this volume is a fitting tribute to a great logician and will be of assistance to those studying his work and developing his ideas in the future. The editors wish to thank the editors of the series, especially the current Managing Editor Anand Pillay, for their support and advice. We also thank Sharon Berry for editorial assistance. Solomon Feferman Charles Parsons Stephen G. Simpson

GENERAL

¨ THE GODEL EDITORIAL PROJECT: A SYNOPSIS

SOLOMON FEFERMAN

The final two volumes, numbers IV and V, of the Oxford University Press edition of the Collected Works of Kurt Gödel [3]–[7] appeared in 2003, thus completing a project that started over twenty years earlier. What I mainly want to do here is trace, from the vantage point of my personal involvement, ¨ the at some times halting and at other times intense development of the Godel ¨ editorial project from the first initiatives following Godel’s death in 1978 to its completion last year. It may be useful to scholars mounting similar editorial projects for other significant figures in our field to learn how and why various decisions were made and how the work was carried out, though of course much is particular to who and what we were dealing with. My hope here is also to give the reader who is not already familiar with the ¨ Godel Works a sense of what has been gained in the process, and to encourage ¨ dipping in according to interest. Given the absolute importance of Godel for mathematical logic, students should also be pointed to these important source materials to experience first hand the exercise of his genius and the varied ways of his thought and to see how scholarly and critical studies help to expand their significance. Though indeed much has been gained in our work there is still much that can and should be done; besides some indications below, for that the reader is referred to [2]. ¨ §1. Early initiatives and serendipitous events. In the first years after Godel died, there was considerable discussion in the Association for Symbolic Logic as to how best to pay tribute to the greatest logician of our time, and to do it in a way that would have scientific and historical value as well. In 1979, Hilary Putnam, then president of the Association, appointed a committee consisting of George Boolos (chair), Burton Dreben and Warren Goldfarb, ¨ whose aim was to produce an edition of Godel’s publications as well as to Reprinted from The Bulletin of Symbolic Logic, vol. 11, no. 2, 2005, pp. 132–149. ¨ The material for this article was presented under the title, “Godel on the installment plan” to ¨ a special session on Godel and Mathematical Logic in the 20th Century at the annual meeting of the Association for Symbolic Logic held at Carnegie Mellon University, Pittsburgh, May 19–23, 2004. Kurt Gödel: Essays for his Centennial Edited by Solomon Feferman, Charles Parsons, and Stephen G. Simpson Lecture Notes in Logic, 33 c 2010, Association for Symbolic Logic

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see if further publishable materials could be extracted from his Nachlass at the Institute for Advanced Study in Princeton. Unfortunately, the faculty member at the Institute who had been assigned the responsibility of dealing ¨ with the Godel Nachlass failed to respond to all inquiries, so the committee was not able to make any progress on that front. Then when I became president of the Association in 1980, we received the disheartening news that a group in Vienna had initiated the production of two volumes, one in German ¨ which would include Godel’s doctoral dissertation together with considerable personal correspondence and memorabilia, and the second in English which would include his complete published works. Since the impression we were given was that they were well advanced in this venture, we decided it would be a mistake to pursue a competitive publication; so, rather unhappily—but with our offer to assist the Viennese in whatever way possible—we threw in the towel. But then a sequence of serendipitous events occurred that succeeded in reviving our project. First of all, a (then young) set-theorist named John Dawson at Penn State in York had come across some minor publications of ¨ Godel that had been overlooked in all the published bibliographies, and in researching the matter, he decided to prepare a complete annotated bibliography. In the process, this bold fellow conceived the idea of writing a biography ¨ of Godel, and he made serious first steps in that direction, including making contact with appropriate parties in Vienna. Dawson’s work came to my attention through an announcement in the Notices of the American Mathematical Society, and we began a correspondence about his efforts. Much to my surprise, one response he received from the Austrian Academy of Sciences ¨ seemed to imply that not only was the Viennese initiative for a Godel edition not far advanced, but also that the plans for it were much more restricted than we had initially been given to understand. As an aside—just to give you an ¨ idea how things can go in this business—Dawson’s biography of Godel [1] was not published until 1997 and the Viennese edition [9], [10] did not end up hitting the stands until 2002. In the winter of 1982 when the possibility of renewing our own efforts thus opened up, it happened that Jean van Heijenoort (“Van”) and Gregory Moore were both visiting Stanford, Van to escape the rigors of the Cambridge winter and Moore to continue his historical research on Cantor’s continuum problem and the development of the method of forcing. As president of the Association, I consulted both of them about undertaking preparation of a ¨ comprehensive edition of Godel’s works under the aegis of the ASL and both urged me to pursue that. I then discussed this with colleagues elsewhere and received further strong encouragement to renew the project. Then the main question became, who should lead the effort? I had asked van Heijenoort if he


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would take on that responsibility; Van, who had spent some ten years of his life working on the source book in mathematical logic, From Frege to Gödel [12] demurred, saying instead that I should do it. Everyone else I asked either said they were too busy or felt that they lacked the confidence for that kind of editorial, historical and scholarly work, or both. I also lacked the confidence that came with the extensive experience of the sort that van Heijenoort and Moore had, but both assured me they would give me full assistance if I were to accept the position of editor-in-chief. So I did. They further convinced me ¨ that, given the relatively small number of Godel’s publications and our full knowledge, finally, of the extent of this corpus, we could produce a volume of his published work in short order, say two years (!). In fact, that projected volume turned into two volumes that took eight years altogether to see into print. Also in early 1982 there was a changing of the guard at the Institute in ¨ Princeton, and the new committee in charge of the Godel Nachlass, headed by Armand Borel, proved to be much more responsive. In connection with his biographical work, John Dawson had applied for membership in the Institute for 1982–1983 in order to study the Nachlass, then stored in its basement. Not only was his application approved, he was also invited to catalogue the material, a task that he eagerly accepted. Little did he know what he was in for. His first inspection of what he would have to deal with was overwhelming, stored as it was in ten file cabinets and over fifty cartons, some of them fairly bursting at the seams. Not surprisingly, Dawson’s one year there turned into two, but it was clear from the beginning that the outcome of his work there would dramatically widen the scope of what we could draw on for our edition. Once catalogued, the Nachlass proved to be a gold mine, containing among other things, unpublished manuscripts, lecture notes, notebooks, and, of course, extensive correspondence. Having made a start on the publications, our problem then was how and when to deal with all this additional material; as it turned out, much of this went on in overlapping ways, with sometimes one thing taking priority, sometimes another, sometimes too many things at once. §2. Dealing with the published work; some basic decisions. The first edito¨ rial board of the Godel Works consisted, besides myself, of John Dawson, Stephen Kleene, Gregory Moore, Robert Solovay and Jean van Heijenoort, with Moore as managing editor and copy-editor. Volume I appeared shortly before Van’s tragic death in the spring of 1986. Volume II was in an advanced stage by that time, and Van had already begun turning his attention to Volume III. Then in 1994, the year before that volume came out, Kleene passed away. The first order of business for the newly constituted board was to deal with the published material, and that led to some major decisions that had

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a big effect on the rest of our project. The easiest thing to do for any edition of collected works is to assemble everything in print by the given author, in whatever language it appeared, and reproduce it photographically. We decided instead to print everything anew in a uniform format, and—since this was to be an English edition—to provide facing translations of everything not in English. We also decided that since this would require checking and rechecking the reprinted versions against the originals and vetting and correcting the translations, we should take control of the typesetting process. That appeared to be feasible by means of Donald Knuth’s then relatively new computerized typesetting system TEX, and we found someone in the Stanford area, Yasuko Kitajima, who was both expert in the system and willing to do the work for us.1 One thing we discovered to our surprise and dismay is that once proofread did not mean forever proofread: each iteration required proofreading ab initio, since there were computer devils that introduced random errors in previously checked parts. So, control over the typesetting had its disadvantages as well as advantages. Another basic decision we made early on, in order to make the full body of ¨ Godel’s work and thought as accessible and useful to as wide an audience as possible without sacrificing historical and scientific accuracy, was that each article or closely related group of articles should be preceded by an introductory note elucidating it and placing it in historical context. This was modeled on the introductory notes in van Heijenoort’s source book [12], but ours turned out to vary in length to a much greater extent, from a few lines to substantial essays, sometimes much longer than the item being introduced. Finally, all references in the original articles together with those in the introductory notes were to be unified. ¨ §3. Dealing with the published work. Godel’s publications fall naturally into two parts, chronologically and substantively. The first part, which ended up comprising Vol. I [3], consists of works dating from 1929 through 1936, and ¨ proceeds from his dissertation—in which Godel established the completeness theorem for first order logic—through the incompleteness theorems, to the short note on length of proofs. We decided to include the Vienna dissertation along with its 1930 published version because the former begins with a quite interesting discussion of the significance of the completeness theorem and the nature of its proof that was suppressed in the latter; among other things, one point in it prefigures the incompleteness theorems. The major publication in that volume is of course the 1931 article containing the incompleteness ¨ theorems. Along with that we have Godel’s 1934 lectures at the Institute for 1 In later years, as the mainstream shifted to LAT X or AMS-T X, we remained with T X, E E E and when Kitajima left to do other work, we had some difficulty finding someone versed in the original system to replace her.


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Advanced Study in Princeton on that same subject, which contain some interesting variations of detail and comments. Also of marked interest in Vol. I are the articles on special cases of the decision problem and on intuitionistic logic and number theory. There too are the previously overlooked articles, including several on geometry, that had been unearthed by Dawson. Finally, all of ¨ Godel’s reviews, many of which contain interesting or pointed observations, date from this period. The second part, comprising Vol. II [4], consists of works dated 1938–1974 (there being no publications in 1937). It begins with the first published outlines of the great proofs of the consistency of the axiom of choice and the continuum hypothesis with the axioms of set theory, followed by their exposition in the 1940 monograph (reproduced in full with corrections.) After that we move ¨ on to the 1944 article on Russell’s mathematical logic—in which Godel first advanced his Platonist philosophy of mathematics—and then to the 1946 Princeton bicentennial remarks on the notion of ordinal definability. That is ¨ followed by “What is Cantor’s continuum problem?” in 1947, in which Godel’s set-theoretical Platonism is made more specific by its application to the major undecided set-theoretical problem. (Its republication in 1964—also included in Vol. II—contains interesting revisions, and remarks on the significance of Cohen’s independence results found the year before.) The years 1949–1952 bring three articles on new solutions of Einstein’s field equations for general relativity theory and the philosophical implications of the possibility of “time travel” into the past. After a break of six years without publications we come to ¨ Godel’s 1958 Dialectica article on an extension of finitism via a quantifier-free functional interpretation of Heyting Arithmetic, a piece dedicated to Paul ¨ Bernays on his 70th birthday. A translation and revision by Godel of that article, initially slated to be published in Dialectica in 1968 for Bernays’ 80th ¨ birthday, was found in marked-up proof sheets in Godel’s Nachlass; he was apparently dissatisfied with the philosophical aspects of the interpretation, and was reworking the discussion of those aspects up until 1972. This version, ¨ revised as far as it was taken by Godel, only saw the light of day in Vol. II of our edition. In addition, we included three notes on the incompleteness theorems that were appended to the proof sheets of the revised Dialectica article. Vol. II ¨ concludes in 1974 with a remark by Godel lauding non-standard analysis as “the analysis of the future.” §4. Dealing with the unpublished work in Volume III. Having reached this point, our next step was to deal with the unpublished articles and texts of lectures found in the Nachlass. As I said, van Heijenoort had already started on this when his life was taken in 1986. In the immediately following years, Kleene decided not to continue and Moore was drawn away by work on the gargantuan Bertrand Russell project at McMaster University, so a new editorial board had to be constituted for Vol. III [5]. This consisted of John

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Dawson, Warren Goldfarb, Charles Parsons, Robert Solovay and myself. In addition, Cheryl Dawson took over from Moore the absolutely essential role of managing editor and copy editor. With the basic format set as in Vols. I and II, here our basic decision was what to select from the available material. We settled on the following criteria for inclusion: (1) The manuscript had to be sufficiently coherent. (2) The text was not to duplicate other works substantially in content and tone. (3) The material had to possess intrinsic scientific interest. ¨ We were also guided in part by two lists prepared by Godel, entitled “Was ¨ ich publizieren konnte.” In some cases it was quite clear what the items in those lists referred to, in other cases less so. But we did not feel bound to restrict ourselves to those items. One of the former items was the 1972 version of the Dialectica article already included in Vol. II; also listed were the three notes on incompleteness that had been appended to its proof sheets and that were included in Vol. II as well. Of course the question has to be asked ¨ what Godel would not have wanted published. Indeed, one item, a supposed disproof of the continuum hypothesis. that he had submitted for publication ¨ in 1970 was withdrawn by Godel when an error was found in a key argument. Nevertheless, we decided to include that because we felt there was still much to be learned from the approach taken therein. ¨ Another concern was that Godel would surely have wanted to make revisions in the items he thought worthy of publication, just as he had kept reworking the 1972 version of the Dialectica article. Here, as we shall see, our problem was compounded in certain cases by the existence of multiple drafts of the same article. A final problem was that some of the material had portions, sometimes substantial, written in the Gabelsberger shorthand system; how we dealt with that will be described below in connection with the ¨ transcription of Godel’s notebooks. ¨ For readers familiar only with Godel’s main publications, here, with brief annotations, are some (but by no means all) of the interesting items that we included in the rich and varied Volume III of the Collected Works (cited with stars as they appear there). • “The present situation in the foundation of mathematics.” This was ¨ the text for a lecture that Godel gave to a meeting of the Mathematical Association of America in 1933 during his first visit to the United States and the Institute for Advanced Study. After describing the problem of foundations to be that of “avoiding the paradoxes [while] retaining all of mathematics”, he says that this has been solved in a completely satisfactory way by axiomatic set theory. But then he says, surprisingly, that the set-theoretical axioms “necessarily presuppose a kind of Platonism, which cannot satisfy any critical mind and which does not even produce


•

•

•

•

•

•

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the conviction that they are consistent.” The final part of the lecture is devoted to Hilbert’s program and the possibilities of overcoming its limitations by intuitionistic foundations of mathematics. (*1933o) A second, related, item was a lecture to Edgar Zilsel’s seminar in Vienna (*1938a).2 This is notable for its pursuit of several possibilities for a revised Hilbert program part of which is a precursor of later work by ¨ Godel and some of which anticipated work by others. In particular he sketches there a quite interesting reinterpretation of Gentzen’s consistency proof for arithmetic in terms of what has since been called the no-counter-example interpretation as later developed by Kreisel in 1951; cf. William Tait’s article [11] in this volume for an analysis. We included two interesting lectures on the consistency of AC and GCH, ¨ the first in Gottingen (*1939b), and the second at Brown University ¨ (*1940a) after Godel had emigrated to the United States. The first is an exceptionally clear exposition behind the ideas of his relative consistency proof using constructible sets. The second gives an alternative approach ¨ which Godel described as related to Hilbert’s earlier failed attempt to prove CH, though Solovay, who wrote the introductory note, judged the connection to be tenuous. An item that we could not date but that was clearly considered for publi¨ cation by Godel was an untitled article, probably prepared for a lecture. Based on its contents, we called it “[[Undecidable diophantine proposi¨ tions]]” and dated it *193?. In this text Godel proves that diophantine problems of the form ∀ . . . ∃ . . . (p = 0) with p a diophantine polynomial are recursively undecidable. This work was unknown to those who later worked on undecidable ∃ . . . (p = 0) diophantine problems. The Gibbs lecture (*1951). Two philosophical consequences of the incompleteness theorems are drawn: First, either mind infinitely surpasses any finite machine or there are absolutely unsolvable problems, and, second, each of these disjuncts “are very decidedly opposed to materialistic philosophy.” Arguments favoring the first disjunct are given. “Is mathematics syntax of language?” (two of six drafts, *1953/9) offers direct and full criticisms of “linguistic” accounts of the foundations of mathematics as developed by the logical positivists. These drafts were prepared for Paul Schilpp’s Library of Living Philosophers volume ¨ devoted to Rudolf Carnap but, in the end, Godel made no contribution to it. He seems not to have been fully satisfied with any of the drafts, and he may also have held back from publication due to his concern with “widely held prejudices” of the time. “The modern development of the foundations of mathematics in the light of philosophy” (*1961/?) deals with Left (skepticism, materialism,

2 The

notes for this lecture were entirely in Gabelsberger, transcribed by Cheryl Dawson.

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positivism, empiricism, pessimism) vs. Right (spiritualism, idealism, the¨ ology, apriorism, optimism) in philosophy. Godel inveighs against the leftist conception of mathematics and finishes with a Husserlian turn.3 ¨ • An ontological proof of the existence of God, told by Godel to Dana ¨ Scott when he thought he was dying. Godel later told Oskar Morgenstern that he hesitated to publish it, even though he was satisfied with the proof, because people might think he believed in God. (*1970) • Axioms for scales of functions and the proof that the cardinal of the continuum is ℵ2 , submitted to Tarski for publication in the Proceedings of the National Academy of Sciences (*1970a). Martin and Solovay found ¨ a key error in the argument, after which Godel withdrew it. The note *1970b uses modified axioms to prove that the cardinal of the continuum is ℵ1 ; this was never published or sent. Item *1970c is a letter to Tarski ¨ apologizing for the submitted note. Godel says he had been ill and was affected by drugs when working on it; the letter may never have been sent. §5. Dealing with the correspondence. When Solovay decided to retire from the project following the completion of Vol. III, his place was taken by Wilfried Sieg for Volumes IV and V; also John Dawson joined me as co-editor-in-chief for these last two volumes, [6] and [7]. Besides the two of us, the new editorial board thus consisted of Warren Goldfarb, Charles Parsons and Wilfried Sieg; Cheryl Dawson agreed to continue in the increasingly demanding job as managing editor. The basic problem faced with those volumes was that of ¨ selecting from the overwhelming extent of Godel’s correspondence, consisting of approximately 3500 items in 219 folders. In order to make this manageable our basic decisions were to: (1) Publish primarily the scientific correspondence. (2) Include only items that possess intrinsic scientific, philosophical or his¨ torical interest, or illuminate Godel’s thoughts or his relations with others.4 These decisions allowed us to whittle down to fifty correspondents; even so, each of volumes IV and V, consisting of correspondence, facing translations where necessary, introductory notes and ancillary materials ran to over 660 pages. Names of the twenty-one correspondents in Vol. IV go from A to G. But the exchange with Paul Bernays, ranging from 1930 to 1975, alone takes up almost half this volume (300 pages, including introductory note and facing translations). It covers a rich body of logical and philosophical material including 3 Transcribed

from the Gabelsberger by Cheryl Dawson. is not at all to say that this is all that may be of interest in the correspondence; indeed, there may be much that we did not include that could reward further study on other grounds, both personal and historical. 4 This


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the incompleteness results and Hilbert’s program; the metamathematics of set theory; Gentzen and proof theory; the limits of finitism and Kreisel’s work on that; type-free systems; foundations of category theory; philosophy of mathematics; Friesian and neo-Friesian (Nelson) schools of philosophy; the pro¨ posed translation/revision of Godel’s 1958 Dialectica article; Bernays’ proof of transfinite induction up to ε0 ; limits of finitism revisited; and Gentzen’s “origi¨ nal” consistency proof. One of the gems is Godel’s put-down of Wittgenstein’s book on the foundations of mathematics (30 October 1958): “I also read parts of it. It seemed to me at the time that the benefit created by it may be mainly that it shows the falsity of the assertions set forth in it.” As a footnote he added: “and in the Tractatus (the book itself really contains very few assertions).” Among other correspondents of interest in this volume are Heinrich Behmann, William Boone, Rudolf Carnap, Alonzo Church, Paul Cohen, Burton ¨ Dreben, Paul Finsler and Godel’s mother Marianne. To give a taste, here is a brief sampling from among these. ¨ The first letter to Cohen found in Godel’s Nachlass is a handwritten, messy draft dated 5 June 1963. We do not know what was actually sent, but may assume it contained some version of the following laudatory passage:5 Let me repeat that it is really a delight to read your proof of the ind[ependence] of the cont[inuum] hyp[othesis]. I think that in all essential respects you have given the best possible proof & this does not happen frequently. Reading your proof had a similarly pleasant effect on me as seeing a really good play. ¨ But the follow-up correspondence was largely devoted to Godel’s suggested revisions of the announcement Cohen had submitted to the Proceedings of the National Academy of Sciences; that dragged on, to Cohen’s increasing discomfort. In 1966, Church was to give a talk at the Moscow meeting of the ICM at ¨ which Cohen would receive the Fields Medal, and he asked Godel whether there was anything that should be credited to him. In a response formulated ¨ for inclusion in Church’s talk, Godel wrote (29 September 1966) that ¨ he [Godel] only had a proof of the independence of the axiom of constructibility in type theory, which, he believed, could be extended to an independence proof of the axiom of choice. But, due to a shifting of his interests toward philosophy, he soon afterwards ceased to work in this area, without having settled its main problems. The partial result mentioned was never worked out in full detail or put in form for publication. About this unpublished work, more in the next section. 5 Paul Cohen refused to let us use his part of the correspondence and did not share the letters ¨ from Godel in his possession.

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The correspondence with Dreben between 1963 and 1970 first concerned a crucial error in the proof of the fundamental theorem in Herbrand’s thesis that had been discovered through work with Peter Andrews and St˚al Aanderaa, and corrected a few years later by Dreben with John Denton.6 It turned out ¨ that Godel had anticipated this in his unpublished notes from the early 1940s, as was established by our project years later (see the next section). Another ¨ part of the correspondence had to do with Godel’s claim from the 1930s to have established decidability of the ∀∀∃ class with equality. As later shown by Goldfarb, this proved to be wrong. ¨ Godel’s letters to his mother are of a very special quality. In the ones chosen for our volume he patiently, lucidly and poetically explains his personal ideas about some philosophical and spiritual matters.7 To quote from one (27 February 1950): You are right about sadness: If there were a completely hopeless sadness, there would no more be anything beautiful in it. But I think that from a rational point of view there cannot be any such thing at all. For we understand neither why this world exists, nor why it is constituted just as it is, nor why we are in it, nor why we were born in just these and no other external circumstances. Why then should we fancy that we know precisely the one thing for sure, that there is no other world and that we never were nor ever will be in another? And in answer to his mother’s question whether they would see each other in a hereafter, he wrote (23 July 1961): About that I can only say the following: If the world is rationally organized and has a sense, then that must be so. For what sense would it make to bring forth a being (man) who has such a wide range of possibilities of individual development and of relations to others and then allow him to achieve not one in a thousand of those? . . . But do we have reason to assume that the world is rationally organized? I think so. For the world is not at all chaotic and capricious, but rather, as science shows, the greatest regularity and order prevails in all things; [and] order is but a form of rationality. ¨ Also in this volume is the little gold mine familiar to Godel scholars and known as the “Grandjean questionnaire”. In 1974, Burke D. Grandjean, then ¨ a doctoral student in sociology, sent Godel a sheet of questions about his early ¨ life and influences. At some point, Godel dutifully filled it out, but he never returned it. Among the responses is one that has been difficult to square with 6 It was Peter Andrews, while still a graduate student working with Alonzo Church, who first convinced Dreben that there was a problem with Herbrand’s proof. See his Herbrand Acceptance Speech, Journal of Automated Reasoning, vol. 31 (2003), pp. 169–187. 7 Further excerpts from others of Godel’s ¨ letters to his mother are to be found in [10].


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other evidence, namely his assertion that he had held the philosophical view of mathematical realism since 1925. And as to religion, he said that his view was “theistic not pantheistic (following Leibniz rather than Spinoza).” Among the exchanges of interest in Vol. V (Correspondence H–Z), are those with Jacques Herbrand, Arend Heyting, Karl Menger, Ernest Nagel, Emil Post, Wolfgang Rautenberg, Abraham Robinson, Bertrand Russell, Paul Schilpp, Thoralf Skolem, Alfred Tarski8 , Stanislaw Ulam, Jean van Heijenoort, John von Neumann, Hao Wang, and Ernst Zermelo. To whet the appetite, here are just a few piquant, more personal, excerpts. ¨ Godel and Heyting had a lengthy exchange during the period 1931–1933 concerning a proposed collaboration on a book surveying (then) recent developments in the foundations of mathematics, but that came to nothing. The correspondence lapsed until January, 1969 when the following came from Heyting: I am writing in the name of the editorial committee of the series “Studies in Logic”. We were told that you consider the publication of your collected works. We are convinced such a publication will be very useful and we shall be happy to publish the book in our series. . . . I take this opportunity to send you my best wishes for 1969. ¨ Godel’s response came a couple of months later: Thank you very much for your letter and New Years’ Wishes. I have so far never been considering an edition of my collected works. In fact, I am very doubtful about the usefulness of such a project, since practically all my papers (and, at any rate, all of my important papers) are readily available . . . There are only a few notes in the “Ergebnisse eines mathematischen Kolloquiums”, edited by K. Menger, which, perhaps are hard to get. But I think they are, at present, more of a historical and biographical, than of a logical interest. (So much for all our work.) ¨ Ernest Nagel’s 1957 correspondence with Godel concerned his prospective publication with James R. Newman of Gödel’s Proof. Nagel and Newman wanted to include as appendices to their exposition an English translation of ¨ Godel’s 1931 incompleteness paper together with his 1934 IAS lectures on that subject. In the course of trying to arrange this and in collateral correspondence ¨ with Allan Angoff at NYU Press, Godel imposed both financial and editorial ¨ conditions. In Nagel’s final letter on the matter to Godel before abandoning the proposal, he wrote: 8 Incidentally, Tarski was the only one of Godel’s ¨ correspondents in these volumes, other than his mother, that he addressed with the personal “Du” when writing in German.

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According to the information Mr. Angoff sent me . . . you stated two conditions for giving your consent to reprinting your papers in our book: the first related to financial matters; the second required that the manuscript of our expository essay be submitted to you for your approval. Now, as far as I know, your financial conditions never constituted serious obstacles . . . . The real hitch arose over your second condition. I must say, quite frankly, that your second stipulation was a shocking surprise to me, since you were ostensibly asking for the right to censor anything of which you disapproved in our essay. Neither Mr. Newman nor I felt we could concur in such a demand without a complete loss of self-respect.9 That ended that matter. ¨ Emil Post made Godel’s personal acquaintance at a meeting of the AMS ¨ in New York in October 1938. Soon after he wrote Godel, apropos his own anticipations of the incompleteness theorem, I am afraid that I took advantage of you on this, I hope but our first meeting. But for fifteen years I had carried around the thought of astounding the mathematical world with my unorthodox ideas, and meeting the man chiefly responsible for the vanishing of that dream rather carried me away. . . . As for any claims I might make perhaps ¨ the best I can say is that I would have proved Godel’s Theorem in ¨ 1921—had I been Godel. In a follow-up letter, Post enlarged on his ideas, and concluded by apologizing ¨ for his “egotistical outbursts”. Godel responded graciously. ¨ Godel first met Abraham Robinson during the latter’s visit to Princeton University in 1960, the same year that Robinson realized his method to create non-standard analysis. Much of their correspondence between 1971 and ¨ 1974 concerned that subject, for which Godel had very high hopes. In his remarks included in Robinson’s book, as noted above, he lauded it with the expectation that it would be “the analysis of the future” (cf. [4] p. 311). But ¨ in his correspondence with Robinson, Godel thought more specifically that it would form a necessary part of his program to find new axioms to settle number-theoretic problems; Robinson, however, sought to dissuade him of that idea by pointing out conservation results for non-standard over standard results. In the latter part of their correspondence there are several references ¨ to Godel’s hopes of bringing Robinson to the IAS as his successor, but they were put in question when it was learned that Robinson had been diagnosed ¨ with pancreatic cancer. Godel wrote him as follows in March, 1974. 9 Ironically, the book contained several now-well-known errors (both in the Godel ¨ coding and ¨ Rosser’s strengthening of the first incompleteness theorem) that Godel might have caught had he been given the opportunity to vet the manuscript.


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In view of what I said in our discussions last year you can imagine how very sorry I am about your illness, not only from a personal point of view, but also as far as logic and the Institute for Advanced Study are concerned. As you know I have unorthodox views about many things. Two of them would apply here: 1. I don’t believe that any medical prognosis is 100% certain, 2. The assertion that our ego consists of protein molecules seems to me one of the most ridiculous ever made. I hope you are sharing at least the second opinion with me. ¨ followed with Robinson died a few days after this letter was written.10 Godel two letters of condolence, one to Dan Mostow, then chair of Mathematics at Yale, and the other, in quite heartfelt tones, to Mrs. Robinson. ¨ The correspondence between Godel and John von Neumann, which extends with some lengthy gaps from 1930 to 1956 (the year of von Neumann’s death) begins and ends with some remarkable technical discussions. Once ¨ von Neumann learned of Godel’s first incompleteness theorem, he was drawn to conclude the second theorem, on the unprovability of consistency. But ¨ Godel had already sent in an abstract, announcing that theorem as well, so von Neumann wrote that “[a]s you have established [the theorem] as a natural continuation and deepening of your earlier results, I clearly won’t publish on this subject.” Following that, they had an exchange in which they differed on the significance for Hilbert’s program of the second incompleteness theorem, with von Neumann arguing strongly that the program was essentially doomed ¨ as originally conceived while Godel thought it was possible that there are finitary proofs that cannot be formalised in the systems considered. But by 1933, ¨ Godel came to the same conclusion as von Neumann. ¨ Godel’s final letter to “Lieber Herr v. Neumann” (20 March 1956) was written to him at Walter Reed Hospital where von Neumann was being treated for bone cancer. In it, after expressing his deep concern and wishes for the ¨ improvement of his condition, Godel raised a problem concerning feasibility of computations which he thought “would have consequences of the greatest significance. Namely, that the thinking of a mathematician in the case of yes-and-no questions could be completely replaced by machines, in spite of the unsolvability of the Entscheidungsproblem.” His question was recognized many years later to be a precursor of the currently open P = NP problem. There is no extant response from von Neumann. He died in the hospital the following year, in August, 1957. ¨ ¨ Other than a few letters written on Godel’s behalf, Ernst Zermelo is Godel’s final correspondent represented in Vol. V of the Collected Works. Zermelo ¨ thought he had found a gap in Godel’s proof of the incompleteness theorem, ¨ but it was due to a confusion on his part between truth and proof. Godel 10 And

¨ four years before Godel’s own death in early 1978.

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tried, respectfully and patiently, to straighten him out, but without success. Their correspondence broke off with that. ¨ One thing that Godel scholars will find particularly valuable in Vol. V ¨ comes toward the end: it is the finding aid for the Kurt Godel Papers in the Manuscripts Division of the Department of Rare Books and Special Collections of the Princeton University Library. Running to almost one hundred pages, it was prepared by John Dawson on the basis of his 1984 catalogue of ¨ Godel’s Nachlass, with some revisions in the late 1990s by Rebecca Schoff and Barbara Volz; further revisions in the finding aid as it appears in Vol. V are by ¨ Cheryl Dawson, incorporating references to the microfilm edition of Godel’s Nachlass. §6. Dealing with the notebooks. The material that attracted perhaps the ¨ most interest at the beginning of our project were Godel’s notebooks, of which there are over one hundred altogether. Those of prima facie scientific or philosophical interest were classified by him in subseries, labelled as follows: — “Arbeitshefte” (mathematical workbooks, sixteen in number) — “Logic and foundations” and “Results on foundations” (reports of results by others, six of the first and four of the second) — “Max” and “Phil” (philosophical notebooks, fifteen in number, with another one missing). In addition there were three theological notebooks, one of which was missing. The notebooks of greatest potential interest to us were the Arbeitshefte and the philosophical notebooks. These are almost entirely in Gabelsberger, one of two competing German shorthand systems in widespread use during the early part of this century. Among those who used this script regularly in addition ¨ ¨ to Godel were Heidegger, Husserl, Schrodinger and Zermelo. However, the Gabelsberger system was officially superseded by a unified script in 1926 and thus by 1982 there were very few people left who knew it. We were eager to decipher those notebooks, because their contents could be gleaned from ¨ an index with 55 headings, that Godel had prepared in longhand for his Arbeitshefte, among whose tantalizing entries are the following:11 1a. Corrections to Herbrand. 3. Attempt [to prove] the consistency of analysis. 4. Ordinals in analysis. 4 . First ordinal number not constructible in an[alysis]. 5. Consistency [of ] Quine [’s system]. 11. Consistency of analysis. 15. Consistency of ¬(p)(p ∨ ¬p). 11 A shorter, simplified list in a different order was given in Vol. I, p. 28. The listing here follows ¨ Godel in his partial index of the Arbeitshefte. An English translation of the complete list of 55 items is to be found in Appendix A of Dawson and Dawson [2].


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20. Computable functions of finite type. 21. Souslin’s problem. 22. Independence of the continuum hypothesis according to Brouwer’s method. 27. Analytically measurable functions, with an application to the proof program for the independence of the continuum hypothesis. 31. Existence of undecidable arithmetic propositions for the system of all ordinal logics (for ordinal numbers of the second class that are constructible (in the strict sense)). 32. Independence of the axiom of choice; For Zermelo set theory; Systems of representation; Consistency of the existence of lawless sets. 35. Attempt to resolve the antinomies by means of a notion of freedom from circularity. 50. Absolute definability. 51. Principle for constructing axioms of infinity (Mahlo). The contents of some of these, e.g., numbers 20 and 50 could be guessed at ¨ from published work of Godel, but we were eager to decode the other items, especially numbers 22, 27 and 32. Of course, in view of Paul Cohen’s independence results, we expected them to be of primarily historical interest. ¨ Moreover (as noted above in his correspondence with Church), Godel had never claimed to have obtained the independence of the continuum hypothesis, and only the independence of the axiom of choice from type theory (or Zermelo set theory). The problem was to find someone within convenient working distance who could help transcribe the shorthand. With good fortune, in the very first year of our project we were led to Hermann Landshoff, a retired photographer living in New York City who had learned Gabelsberger as a student in Germany and was very willing to help. He knew no mathematics but was able to train Cheryl Dawson in the script; Cheryl had a background in both mathematics and German that allowed her to transcribe the contents of the notebooks in a form that could then be digested by experts. ¨ It also helped that a Gabelsberger textbook was found in Godel’s Nachlass; it contained some special signs that he had designed for his own use. Then, out of the blue, a couple of years after Landshoff and Cheryl Dawson had immersed themselves in the notebooks, I received an inquiry from a student named Tadashi Nagayama in Tokyo, who was a student of a student of Gaisi Takeuti. Nagayama’s teacher had been given copies of ex¨ tracts from Godel’s notebooks by Takeuti, in the hope that someone could be found to transcribe them. These were passed on to Nagayama, whom Takeuti also supplied with a copy of the same Gabelsberger textbook used ¨ by Godel. Nagayama diligently set to work mastering the system, after which he wrote me asking whether he could assist on the project. The

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whole thing was a bit hard to believe, but after testing him with some passages that had already been worked out by Hermann Landshoff and Cheryl Dawson, we recognized that he could indeed be a help. Nagayama came to Stanford in 1985 under support from our grants, and worked full time for about six years, both in collaboration with Cheryl on the Gabelsberger part of the project and in many other ways. During this period, his wife Misao Nagayama joined him and entered graduate study in mathematics at Stanford, finishing with a Ph.D. thesis on Boolean algebras with Philip Scowcroft. Afterward, she returned to a position in Tokyo and Tadashi moved on to do free-lance technical translation—surely first-rate if our experience is any guide. In this way, substantial portions of the notebooks were transcribed into German, from which they were translated into English. That’s the good news of the story. The bad news is that what we have as a result is not at all suitable for publication in its present form; after extensive discussion the editors judged it would take a considerable further investment in time, energy and funding to make that material widely available—time, energy and funding that we could no longer draw on either individually or as a group. That is one promise we have thus, regrettably, had to break. For that reason we decided in 1995 to ¨ bring our project to a conclusion with the final two volumes of the Godel Works as described above. To wrap up, I know many of you will want to know what, in particular, was gleaned from the notes on the independence of the Axiom of Choice. Well, those extracts were put in the hands of Robert Solovay and Donald A. (Tony) Martin, and they tried very hard to connect the dots of what formally looked like a topological model for the negation of the Axiom of Choice. The best that they could come up with was that it looks like a form of forcing, but they were unable to extract a coherent proof of its independence from ¨ Godel’s working notes. Only item 1a, “Corrections to Herbrand”, received as much attention, in that case from Jean van Heijenoort and Warren Goldfarb. (Cf. Goldfarb’s article [8].) They were excited to see that, unbeknownst to ¨ Dreben and his co-workers years later, Godel had recognized the problems with Herbrand’s proof of his “théoreme fondamentale”, and had worked out a fix. No doubt there are many more gems to be unearthed, but we’ll have to bequeath them to those with the capacity and inspiration to carry on this work. The challenges of dealing with all the left-over material (including but by no means restricted to the notebooks) is well described by John and Cheryl ¨ Dawson in their article “Future tasks for Godel scholars” [2], to be found in this volume. §7. “Without which . . . and without whom . . . we could not have . . . ”. In the ¨ preface to each volume of the Godel Collected Works we have expressed our extensive indebtedness to the institutions and individuals whose contributions


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in one way or another were indispensable to the success of our project. As prefaces tend not to be read, I want at least to single out a few of the most important of these, to repeat here our gratitude: • The ASL for sponsoring our project throughout and for initial and final financial support12 • The Sloan Foundation and the National Science Foundation for grants to support the work on Volumes I–III • The Sloan Foundation for grants to support the work on Volumes IV and V and for a special grant in support of the preservation microfilming ¨ of Godel’s Nachlass • C. Ward Henson and Charles Steinhorn for administering those grants through the ASL • The Institute for Advanced Study and the Firestone Library of Princeton University for help in dealing with the organization and relocation of the Nachlass • Yasuko Kitajima and Bruce Babcock for the TEX work on the volumes • All our colleagues who contributed introductory notes alongside those of the editors • Oxford University Press, publisher of the volumes • Stanford University and Penn State York for work space and staff support. Finally, I wish to express my personal gratitude to all my co-editors over the years—John Dawson from the beginning to the end, and, as detailed in secs. 3–5, Warren Goldfarb, Stephen Kleene, Gregory Moore, Charles Parsons, Wilfried Sieg, Robert Solovay, and Jean van Heijenoort for their editorial work on various of the volumes. My special thanks go to Cheryl Dawson for her work on both the Gabelsberger part of the project and for taking over as managing editor from Gregory Moore following completion of Vol. II. Last but not least, Stefan Bauer-Mengelberg gave us extensive help with the translations from the German in Vols. I–III and with the heavy proofreading; sadly, he passed away in 1996. Acknowledgments. I wish to thank John Dawson, Aki Kanamori, Paolo Mancosu, Wilfried Sieg, Bill Tait and Charles Parsons for their helpful comments on a draft of this article. REFERENCES

[1] John W. Dawson, Jr., Logical dilemmas. The life and work of Kurt Gödel, A. K. Peters Ltd., Wellesley, 1997. 12 As explained in the Preface to Vol. I, seed money to get us off the ground during a difficult early period was provided through the ASL by two donors who wished to remain anonymous; one of them was later revealed to be Julia Robinson, following her death in 1985 (cf. Vol. I, p. xii).

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[2] John W. Dawson, Jr. and Cheryl Dawson, Future tasks for Gödel scholars, The Bulletin of Symbolic Logic, vol. 11 (2005), no. 2, pp. 150–171, also reprinted in this volume. [3] Solomon Feferman et al.(editors), Kurt Gödel, Collected Works. Vol. I. Publications 1929– 1936, Oxford University Press, New York, 1986. [4] Solomon Feferman et al.(editors), Kurt Gödel, Collected Works. Vol. II. Publications 1938–1974, Oxford University Press, New York, 1990. [5] Solomon Feferman et al.(editors), Kurt Gödel, Collected Works. Vol. III. Unpublished essays and lectures, Oxford University Press, New York, 1995. [6] Solomon Feferman et al.(editors), Kurt Gödel, Collected Works. Vol. IV. Correspondence A–G, Oxford University Press, Oxford, 2003. [7] Solomon Feferman et al.(editors), Kurt Gödel, Collected Works. Vol. V. Correspondence H–Z, Oxford University Press, Oxford, 2003. [8] Warren Goldfarb, Herbrand’s error and Gödel’s correction, Modern Logic, vol. 3 (1993), pp. 103–118. ¨ [9] Eckehart Kohler et al.(editors), Kurt Gödel, Wahrheit und Beweisbarkeit. Bd. I. Dokumente ¨ & Hpt, Vienna, 2002. und historische Analysen, Obv ¨ [10] Eckehart Kohler et al.(editors), Kurt Gödel, Wahrheit und Beweisbarkeit. Bd. 2. Kom¨ & Hpt, Vienna, 2002. pendium zum Werk, Obv [11] William W. Tait, Gödel’s reformulation of Gentzen’s consistency proof for arithmetic: The no-counterexample interpretation, The Bulletin of Symbolic Logic, vol. 11 (2005), no. 2, pp. 225– 238, also reprinted in this volume. [12] Jean van Heijenoort, From Frege to Gödel. A source book in mathematical logic, 1879– 1931, Harvard University Press, Cambridge, 1967. (Third printing, with corrections, 1976.) DEPARTMENTS OF MATHEMATICS AND PHILOSOPHY STANFORD UNIVERSITY STANFORD, CA 94305, USA

E-mail: [email protected]

¨ FUTURE TASKS FOR GODEL SCHOLARS

JOHN W. DAWSON, JR. AND CHERYL A. DAWSON

¨ Abstract. As initially envisioned, Godel’s Collected Works were to include transcriptions of material from his mathematical workbooks. In the end that material, as well as some other ¨ manuscript items from Godel’s Nachlass, had to be left out. This note describes some of the unpublished items in the Nachlass that are likely to attract the notice of scholars and surveys the extent of shorthand transcription efforts undertaken hitherto. Some examples of sources outside ¨ ¨ Godel’s Nachlass that may be of interest to Godel scholars are also indicated.

¨ At the time the Godel Editorial Project began in the summer of 1982 the ¨ cataloguing of Godel’s Nachlass had only just begun. Nevertheless, despite limited knowledge of the extent of that collection, enthusiasm and expectations ¨ were high: Volume I of Godel’s Collected Works, which appeared four years ¨ later, confidently declared that “Succeeding volumes are to contain Godel’s . . . lecture notes, as well as extracts from his scientific notebooks.” In the end, volume III of those Works [5], devoted to previously unpublished essays and lectures, contained the texts of a number of individual lectures that ¨ Godel gave on various occasions, as well as those of several manuscripts that he left in relatively finished form, including some items transcribed from his Gabelsberger shorthand. But none of the five volumes ultimately published ¨ contain any of the notes that Godel prepared for his lecture courses at the University of Vienna or at Notre Dame, nor any extracts from the three series of notebooks he compiled on mathematics and philosophy. In particular, to ¨ the great disappointment of the editors and other scholars, details of Godel’s reputed proof of the independence of the axiom of choice in type theory are conspicuously absent. There were several reasons for the editors’ inability fully to realize their intentions: gross underestimation of the time required to do the editorial work; difficulties in reconstructing some of the texts; changes in personnel (in particular, the tragic loss of Jean van Heijenoort on the eve of the appearance of volume I); and eventually, after twenty years of effort, exhaustion of sources of funding. The present note discusses various items that were considered for inclusion in the Collected Works but remain unpublished. Its aim is to explain Reprinted from The Bulletin of Symbolic Logic, vol. 11, no. 2, 2005, pp. 150–171. Kurt Gödel: Essays for his Centennial Edited by Solomon Feferman, Charles Parsons, and Stephen G. Simpson Lecture Notes in Logic, 33 c 2010, Association for Symbolic Logic

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why, in the end, they were left out and to encourage younger scholars to take up where we left off. Some of the items, it appears to us, are too fragmentary to deserve further attention. Others, however, are likely to reward future study. §1. Unpublished items in Gödel’s Nachlass of potential scholarly interest. ¨ Volume V of Godel’s Collected Works contains a detailed outline of the contents of his Nachlass, housed at the Firestone Library of Princeton University. That inventory includes cross-references to a set of preservation microfilms of ¨ the Nachlass that was prepared under a grant to the Godel Editorial Project from the Alfred P. Sloan Foundation.1 The items described below are cited by archival item, box and folder numbers, together with the number(s) of the commercially available microfilm reel(s) on which the items appear.2 They fall into several categories: I. Topical notebooks. The three series of shorthand notebooks mentioned above are likely to be of the greatest interest to mathematicians and philosophers. They are the Arbeitshefte (workbooks), comprising 16 handwritten volumes together with a partial index thereof (items 030016–030034, box 5c, folders 12–28; reels 2 and 3); Resultate Grundlagen (results on foundations), four handwritten volumes plus subject index (items 030115–030119, box 6c, folders 82–86; reel 7); and a series designated Max/Phil (philosophical notebooks), of whose 15 volumes one (volume XIII) is missing (items 030086–030100, box 6b, folders 63–72; reels 5 and 6). In addition, a volume entitled Protokoll (“minutes”), also in shorthand, but with German and English headings, contains some interesting notes on conversations with and lectures by others (item 030114, box 6c, folder 81; reel 7). The Arbeitshefte are much like a physicist’s laboratory notebooks. They record false starts, the working out of ideas, and approaches that might be ¨ pursued, and so have the potential to shed considerable light on Godel’s ways of thinking and working. Altogether the Arbeitshefte comprise 1212 pages, of which 750 have been transcribed in some form. Their topics are ¨ in no particular order, but Godel himself prepared a partial index to the volumes (translated below as Appendix A). In addition, the editors compiled a list of the section headings found within those volumes (reproduced here as Appendix B). Both were used as guides for judging which sections appeared to be of greatest importance for transcription. Transcripts of those sections were then prepared and circulated among the editors and some outside experts in an effort to determine what material might be suitable for publication. The 1 A project carried out in the face of substantial initial opposition from library officials then in charge of the collection. 2 Copies of individual microfilm reels, excluding Godel’s ¨ correspondence, are available for purchase from IDC Publishers, Leiden. For online ordering information see http:// www.ide.nl/catalog/index.php?c=375.


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resulting transcriptions are presently in various stages: some have been typed and vetted, some are pencil drafts that will probably require much revision, and some are accompanied by draft translations. The Resultate Grundlagen, as the title suggests, are statements of foundational results, written up more formally than material in the Arbeitshefte. The four notebooks contain a total of 391 pages, all of which have been roughly ¨ transcribed in some form. They appear to consist of Godel’s own results, in contrast to another set of six notebooks, entitled Logik und Grundlagen ¨ (“Logic and Foundations”), in which Godel primarily recorded results obtained by others. A list of the headings found in the Resultate notebooks is given in Appendix C, and an index to the topics in the Logik und Grundlagen series in Appendix D. The Max/Phil series of philosophical notebooks proved more difficult to survey. Of their 1576 pages, 715 have been transcribed in some form, and ¨ a number of short excerpts relating to Godel’s “Ontological Proof” were published in volume III of the Collected Works. Some idea of the contents of those notebooks may be gleaned from the many names that appear in longhand. (It is a principle of the Gabelsberger system that proper names ¨ should be rendered in longhand. Godel, however, did not always observe that rule.) A list of those names and where they occur was prepared, which may prove useful to future scholars. ¨ II. Notes for seminars and lecture courses. During the years 1931–32, Godel, then a Privatdozent, was responsible for choosing the topics and preparing the students for Hans Hahn’s logic seminar at the University of Vienna. Between 26 October 1931 and 4 July 1932 the seminar met in twenty-two weekly ¨ sessions, seven of them featuring talks by Godel himself. Typescript notes ¨ were prepared, copies of which are preserved both in Godel’s Nachlass (items 004030 and 004031, box 7b, folder 16; reel 10) and among Rudolf Carnap’s papers at the University of Pittsburgh. The notes are legible and coherent. ¨ Most of the topics, however, such as Godel’s completeness theorem, are familiar and available in other presentations. For that reason it was decided not ¨ to publish those notes, or even just the parts constituting Godel’s own contributions. Among the latter, however, are discussions of Herbrand’s thesis (session 19, 13 June 1932) and Herbrand’s consistency proof for a fragment of arithmetic (sessions 21 and 22, 27 June 1932 and 4 July 1932). They do ¨ not duplicate Godel’s other writings and may well be of interest to students of Herbrand’s work. ¨ During his years as Privatdozent Godel also gave three lecture courses at the University of Vienna, entitled Foundations of Arithmetic (spring 1933), Topics in Mathematical Logic (summer 1935), and Axiomatic Set Theory (May–June 1937). Notes for the first of those occupy part of a spiral-bound

24


notebook (item 40104, box 7b, folder 25; reel 10), written partly in German longhand and partly in Gabelsberger shorthand, that consists mostly of mathematical symbols. There is no connective text, and the content appears to consist merely of derivations of some elementary theorems of arithmetic. The notes for the 1935 lectures (item 040126, box 7b, folder 31; reel 10) are in Gabelsberger shorthand. Topics include truth tables, predicate logic, Skolem ¨ normal form, the Skolem-Lowenheim theorem, the decision problem, and set theory. They are more extensive than the notes from the 1933 lectures, but still ¨ appear to be sketchy, intended for Godel’s own use. For that reason, as well as the familiar nature of the material, the editors decided against transcribing them. The lectures on axiomatic set theory, also in shorthand, occupy a 58-page notebook (item 040140, box 7b, folder 36; reel 10), the first 17 pages of which ¨ have been transcribed. It was during June of 1937 that Godel carried out the crucial step in his proof of the consistency of the generalized continuum hypothesis relative to Zermelo-Fraenkel set theory, so it was thought that those notes might be of special interest. They, too, however, are very sketchy, with little in the way of connective commentary. There appears to be no mention in them of the consistency results, but further study may be warranted. ¨ Godel lectured on the consistency results both at the Institute for Advanced Study in the fall of 1938 and at Notre Dame the following semester. Notes of the former occupy seven notebooks (items 040150–040154, box 7c, folders 39–43; reel 11), and those of the latter another five (items 040194–040198, boxes 7c and 8a, folders 52–56; reels 11 and 12). Written in English, they are detailed and coherent. They were not included in volume III of the Collected ¨ Works because of their overlap with other published treatments by Godel of the same material. ¨ At Notre Dame Godel also lectured on logic in a joint seminar with Karl Menger. Notes on those lectures are preserved in eight notebooks (items 040209–040218, box 8a, folders 58–65; reel 12), written in English except for a few passages (notably, the examination questions!) in Gabelsberger. The ¨ lectures constitute a basic introduction to logic, presented in Godel’s usual clear style. Volume I, actually the second of the notebooks, appears to be a rewritten, somewhat condensed version of the first (numbered 0). There are some gaps in the page numbering and some breaks in the continuity of the text, but no loose pages. It and the next two volumes are devoted to the development of the propositional calculus, including the completeness proof and a demonstration of the independence of the axioms employed. Volume IV begins with some remarks about Gentzen’s sequent calculus (several pages of which are missing) and goes on to introduce the predicate calculus. Volume V develops that calculus further; completeness is stated but not proved, as is the negative solution for the decision problem for validity. Volume VI


25

introduces the calculus of classes and relations and mentions type distinctions ¨ and Russell’s paradox. The final volume presents Godel’s analysis of that antinomy and briefly mentions the liar paradox. Although the material is standard, the choice and ordering of topics, as well as some of the examples that are discussed, may well be of pedagogical interest. ¨ Godel also delivered two series of lectures at the Institute for Advanced Study following his emigration in 1940. In April of that year, not long after his arrival there, he lectured on the constructible sets, notes for which fill two notebooks in his Nachlass (items 040239 and 040242, box 8a, folders 70 and 71; reel 12); their contents are too similar to his other presentations on that topic to warrant publication. The following year, also in April, he lectured at the I.A.S. on intuitionism. Notes on that topic take up 117 notebook pages (items 040407–040409, box 8c, folders 121–123; reel 15), handwritten in English except for a few shorthand annotations (now transcribed). The lectures form a self-contained introduction to intuitionism that, while not polished, is generally coherent and readable. It breaks off, however, at the ¨ end. Godel begins by saying that in contrast to the intuitionists themselves, who take the meaning of the intuitionistic logical notions as primitive, he believes they “can be defined in terms of simpler and much clearer ones”, and declares that “the chief purpose of these lectures [is] to give such a definition and consequent proof of the intuitionistic axioms”. Some references to these ¨ lectures are to be found in the introductory note in volume III of Godel’s Collected Works to his lecture at Yale on intuitionism, which was given at nearly the same time. The I.A.S. lectures were not published because of their incomplete nature and their overlap with that Yale talk. Scholars of intuitionism, however, will likely wish to examine them in more detail. III. Other shorthand items. Almost all of the shorthand material found ¨ in Godel’s Nachlass was written for his own personal use, rather than for publication or for sharing with others. In addition to the notebooks and notes for lecture courses described above, there are several other categories of such material, to wit: (1) Incidental jottings, such as short comments written on letters, envelopes or manuscripts; notes written in books; lists of various items; and so on. Where they were found to be relevant to items published in the Collected Works, such items were transcribed. Otherwise, they are likely to be of interest mainly to scholars studying the particular papers on which they are found. (2) Drafts of various papers and individual lectures, such as the Vortrag bei Zilsel; “The modern development of the foundations of mathematics in the ¨ ¨ light of philosophy”; and Godel’s lecture at Gottingen on Cantor’s continuum hypothesis, all of which were included in volume III of the Collected Works. Other items are less coherent or less suitable for publication. A prime example of the latter is a notebook (item 040019, box 71, folder 10; reel 9) containing

26


¨ Godel’s draft for a proposed joint publication with Arend Heyting. (For further discussion of that endeavor see Dawson 1997, pp. 83–85, or Charles Parsons’ introductory note to the Heyting correspondence, pp. 27–33 in volume V of the Collected Works.) The fragmentary nature of that notebook ¨ reveals how little progress Godel made toward an organized presentation of his part of the projected volume, and after some initial attempts at reconstruction the editorial committee decided that the chances of making sense of it were too small to warrant the effort. (3) Sheaves of loose bibliographic notes on the writings of particular individuals, such as Leibniz, Husserl, or Wronski. Evidently these will be of interest primarily to those studying the particular person to whom the notes refer. (4) Other loose manuscript notes. Of particular interest here are two sets of pages, one (item 060001, box 11b, folder 1; reel 27) entitled “American constitution and government” and the other (item 060103, box 11b, folder 11; reel 28) “Finitismus; Cohen’sche Methode 1942” (evidently so labeled after 1963). The first has been of interest to legal scholars and others intrigued by ¨ the well-known anecdote about Godel’s attempt at his citizenship hearing to explain to the judge how the U.S. Constitution might permit the establishment of a dictatorship. In response to several queries, we have transcribed portions of those notes, but so far we have found nothing relevant to the citizenship incident. The material appears rather to be just the sort of notes one would expect a candidate for citizenship to compile in preparation for an examination on the Constitution and the structure of the U.S. government. The second was of immediate interest to set theorists, since, together with ¨ some passages from Arbeitshefte 14–15, it appears to contain Godel’s notes on his reputed proofs of the independence of the axioms of constructibility and choice in the framework of type theory. Early on it was transcribed in full, and ¨ as noted in Feferman’s article “The Godel Editorial Project: A synopsis”, the transcript was sent for examination to several eminent set theorists, including Robert Solovay and A. D. (Tony) Martin. But none of those who have ¨ studied the notes have been able to develop a clear picture of what Godel was ¨ thinking. Again, the notes are quite sketchy, having been made for Godel’s own use; and in later correspondence with Wolfgang Rautenberg (reproduced ¨ on pp. 182–183 of volume V of the Collected Works) Godel admitted that on the basis of his “very incomplete memoranda from that time” he himself could “without difficulty” reconstruct only one of the “partial results” he had obtained during the summer of 1942. (See Charles Parsons’ introductory note ¨ to the Rautenberg correspondence for further reflections on Godel’s claims.) Given that admission, and in light of the failure of the experts we consulted ¨ to make much headway in interpreting Godel’s notes, the editors regretfully


27

concluded that that material was not suitable for publication. We will be delighted if future scholars are able to penetrate further. Other archival sources of potential interest to Gödel scholars. Already, schol¨ ars have unearthed some interesting documents relating to Godel in collections outside his own Nachlass. Paolo Mancosu, for example, in his papers [8] and [9], mined the Carnap archives at the University of Pittsburgh, the Wis¨ senschafthistorisches Archiv at the E.T.H. in Zurich, the Felix Kauffmann and Hans Reichenbach archives at Konstanz, and the Heinrich Behmann archive at Erlangen. And Maria-Elena Schimanovich-Galidescu, in her [10], has drawn upon holdings in the archives of the Vienna Rathaus and the Uni¨ versity of Vienna to fill in details of Godel’s Vienna years. In particular, she ¨ has compiled a nearly complete list of the courses in which Godel enrolled at the university. The diaries of Oskar Morgenstern, held at the Perkins Library of Duke Uni¨ versity, were a valuable resource in the writing of Godel’s biography (Dawson 1997), and recently the papers of Karl Menger, containing at least one item ¨ of correspondence with Godel not known to the editors of volume IV of the Collected Works, have been opened to scholars at that same repository. ¨ More recently still, the Firestone Library at Princeton, where Godel’s Nachlass is housed, has made the papers of Alonzo Church available to scholars. An on-line finding aid is now available at http://libweb.princeton.edu/ libraries/firestone/rbsc/aids/church. To date scholars have not been given access to the papers of Hao Wang, held by Rockefeller University. If and when they become available, they may ¨ well be of considerable interest for Godel studies. Further exploration may ¨ perhaps also yield greater insight into Godel’s physical and mental health problems. Whether any medical records of his survive outside the Nachlass remains to be determined. Request. At this time, no decision has been reached regarding the ultimate ¨ disposition of the shorthand transcriptions prepared by the Godel Editorial Project. How best to make them available to interested scholars is a matter on which the editors would welcome readers’ suggestions. Acknowledgments. We thank Aki Kanamori and Solomon Feferman for suggesting improvements to our original draft of this article. Appendix A: Translated Entries from Gödel’s Index to his Arbeitshefte. 1. Continuum [hypothesis using] recursive [functions] VI, 32; VII, 19–21, 28–42, 47–49.2, 52–54; VIII, 5–12 1a. Corrections to Herbrand IV, 29; V, 22, 35–53 (Thm 4: p. 47)

28


2. Consistency of the axiom of replacement VI, 59 3. Attempt [to prove] the consistency of analysis VII, 1–4, 10, 49.3 4. Ordinals in analysis VII, 5–12 4 . First ordinal number not constructible in an[alysis] VIII, 16–25 4 . Finite recursions for ordinals VIII, 13–15; X, 38 5. Consistency [of ] Quine[’s system] VII, 12, −8 – −9 ¨ 6. Schonfinkel VII, 13–14 7. Intuitionistic many-valued logic and modal logic VII, 15–18, −1 – −6, −11 8. Completeness axiom for the calculus of relations (i.e., variable-free functional calculus) VII, 21–22, 24–28 9. Rosser’s consistency proof [for] Church[’s system] VII, 43–47, 49–52, −12, −16 – −18.4 10. Interpretation of intuitionistic logic VII, −12 – −15 (heur.), −18 – −37; IX, 5–15, 41–63, 74–81; X, 5–8; XII, 36–46 Intuitionistic logic of premises IX, 64–72, 82–84 Equality rule IX, 73–74 11. Consistency of analysis VII, −32.3; XI, 34–37; XIII, 56–60 12. Reduction of proofs [by induction] on 0 [to] definitions [by recursion] on 0 VIII, 22 (impossible for usual); IX, 16–17 13. Connection between 0 and the system of finite tuples VIII, 26–32; IX, 25–41; X, 9–25, 30–32, 36–41 14. Definition and proof by induction for schemes of ramification (partial orderings) IX, 18 15. Consistency of ¬(p) (p ∨ ¬p) IX, 85–88; X, 44–55; XI, 1–27 16. Proof of the axiom of extensionality for Σ X, 33–46, 42–43


29

17. Lemma for Gentzen’s consistency proof with choice sequences XI, 28–33, 55–56 18. Uniqueness of the quaternion field XI, 37–41 19. Metrics in unbounded function spaces XI, 41, 46; XII, 26–28 20. Computable functions of higher type XI, 43–46; XII, 28–35, 49–59, 65–74: syst.; XIII, 61–96; XIV, 1–12 21. Souslin’s problem XI, 47–54 22. Independence of the continuum hypothesis according to Brouwer’s method XI, 56–57; XII, 22–26, 46–49, 60–62, 62–65 (Extensionality Axiom), 75–99; XIII, 1–6, 9–29, 31–38 (Functions of class K), 40–42 23. Germs of orderings XI, 58; XII, 13–19 24. Brouwer’s [work in] the [Proceedings of ?] the Academy of Sciences, Amsterdam XII, 3–12 25. [Crossed out] 26. Calculation with “pieces” XII, 51–58 27. Analytically measurable functions, with an application to the proof program for the independence of the continuum hypothesis XIII, 7–9 28. Extendability of elements in schemes of ramification XIII, 42–44 29. Regular sequences of ordinal numbers XIII, 44–46; XVI, 44–48 30. Extension of measures XIII, 46–50 31. Existence of undecidable arithmetic propositions for the system of all ordinal logics (for ordinal numbers of the second class that are constructible (in the strict sense)) XIII, 51 32. Independence of the axiom of choice XIV, 14–34, 92–93 For Zermelo set theory XIV, 34–43; Blue Hill: XV, 26–57 Systems of representation XV, 57–96; XVI, 1–8 Consistency of the existence of lawless sets XVI, 38–40; perhaps Max IV–VI?

30


33. Brouwerian non-Archimedean number systems XIV, 21–23 34. Refutation of the Fregean solution of the antinomies XIV, 44–48 Refutation of other similar solutions XIV, 48–49 35. Attempt to resolve the antinomies by means of a notion of freedom from circularity XIV, 55–57 36. Proof that every primitive recursive function in the system of arithmetic (with + and · as primitive notions) is representable and computable XIV, 50–54 37. Attempt at a consistency proof for the ordering axiom (unsuccessful) XIV, 58–76 Another way (namely, with the help of constructible sets) XIV, 77–85 General considerations about realizations of this kind XIV, 64–71, 89–90 Only beginnings XIV, 86–91; XV, 1–2 38. The formula ¬(p) (¬p ∨ p) can be adjoined consistently to arithmetic (proof by means of undecidable propositions) XIV, 91, 93 39. Functional calculus with inclusion of B and its realizations XV, 3–7 (something wrong here), 13–19 40. Consistency of the axiom of choice (initiation of the proof) XV, 8–11 41. Consistency of the axiom of extensionality XV, 11–13 42. Unprovability of consistency on the basis of few assumptions (wrong) XV, 19–25 43. Remarks on my work about Russell XVI, 9–10 44. Attempt at a consistency proof for ramified type theory (wrong?) XVI, 10–23 45. Aristotelean theory of modal inferences XVI, 23–26 46. Simpler derivation of Simpson’s Rule, Euler’s summation formula, and Peano’s intermediate-value theorem XVI, 26–30 47. Coupled systems in quantum mechanics XVI, 31–32


31

48. Attempt [to formulate] the relativity theory of a classical system (e.g., hydrodynamics) in relation sentences XVI, 33–38 49. Metrization of the space of G sets (respectively, of functions of the 2nd class) so that monotone (resp., functional) convergence implies metric convergence (or conversely) XVI, 42–44 50. Absolute definability XVI, 51–53 51. Principle for constructing axioms of infinity (Mahlo) XVI, 54 52. Theta functions XVI, 55–58 53. Error theory for large angles XVI, 60–71 53 . Bessel’s differential equation (solution by quadrature) XVI, 72–79 54. Impossibility of an absolute (that is reflexive) proof concept within a finite language, and similar [things] XVI, 80–89 55. Comparisons, also memoranda slips among the excerpts ⇒ This index begins only with volume VI → volume IV, p. 26 is about consistency up to the kth step Back of index page 1: Workbook I—Entire program of my research (approximately 100 questions) Back of index page 2: [various page references, then] separation between mathematics and philosophy Back of index page 5: addendum to 1a—Discussion of Herbrand and applicability goes further in volume V, page 79 (in particular Theorem 11 (p. 63), the theorem about: if P and P → Q are satisfiable in a certain finite field, then Q is satisfiable in an estimable field) ¨ Appendix B: Translated Headings from Gödel’s Arbeitshefte. Some of Godel’s workbooks (such as number 3) bear no headings at all; others are divided into many topically labelled sections. The latter are translated here. Negative page numbers refer to material written forward from the back of a volume. Vol. I

Page(s) Translated Heading inside cover

Continuum (work) [This volume appears to consist almost entirely of questions and ideas concerning the continuum hypothesis. There are no headings per se, but a few conspicuous phrases are listed below.]

32


Vol.

II

Page(s) Translated Heading 11 General basis for W1 and W2 24 Resumé about the consistency of the axiom of choice and the generalized continuum hypothesis 32 The general existence theorem for recursive definitions (whether ordinal numbers be introduced in von Neumann’s or Russell’s sense) rests on the axiom of replacement 38 Results of reflections 40 Program [of his research] 41 Skolem and Bernays 62 Consistency of the generalized continuum hypothesis 63 Fundamental mapping of pairs of ordinal numbers to ordinal numbers [In addition, workbook #1 contains 28 pages of algebraic and calculational entries, as well as 10 pages of notes labelled “Conversation with von Neumann, July 17, 1937” dealing mainly with rings and matrix algebra.] Last 15

III IV

[Except for the last 15 pages, this volume contains no headings or page numbers.] The measure problem (1–12) Totally imperfect co-analytic sets (12–15) [The rest of the volume continues the work on the continuum hypothesis begun in volume 1.] [Unlabelled. This volume contains only 14 pages.]

1–3 3– 4 4–5 5–13

14–17 18 19–20 21–22 23–28 29–30

Decision procedure for positive logic Attempt [to prove] the independence of the axiom of choice Directed graphs Representation of recursive functions by means of Diophantine equations with one alternation of quantifier Existence of automorphisms in models Unprovability of consistency Considerations preliminary to p. 23 Different definitions of the concept of “formal system” Proof of consistency up to the N th step False lemmas in Herbrand[’s Thesis]


Vol.

V

VI

Page(s) Translated Heading 30–33 Mostowski (well-ordering [is] independent of [the] ordering [principle]) 34–39 Gentzen 39– 40 Gentzen with choice sequences 41 Formulation of the general theorem 42–45 Second special case 45 Third special case of the theorem 46–51 Proof of the general theorem 50.1–.7 Gentzen’s consistency [proof ] 53–57 Consistency of the axiom of foundation without the axiom of replacement 1–6

Completeness of the system with transfinite rules for the natural numbers 6–12 Transfinite disjunctive rules of inference 13–14 Finitely many truth values in Heyting[’s system] 14–18 Herbrand: more on the false lemmas 18–33 Herbrand’s Thesis 33–35 Attempt to find a counterexample to Herbrand’s lemma (i.e., Theorem II) 35–71 (Proofreading) correction to Herbrand: heuristic consideration 72–73 Church’s [definition of ] pairs 73–78 Herbrand’s axioms equivalent with the preceding 83–93 Infinite ramification schemes −9 – −5 −4 −3 – −2 32–34, 42–45 35– 41 46–58 59–65

VII

33

Continuum hypothesis with recursive functions Turing computability equivalent to that of Herbrand Continuum hypothesis with recursive functions Continuum hypothesis with recursive functions Insertion on absolute undecidability (draft and final copy) Absoluteness in models (continuum hypothesis) Higher transfinite types and unsuccessful attempt to prove the consistency of the ordering axiom

1–6 Łukasiewicz’s many-valued logic 7–8 Russell’s antinomy without negation, with general application of the positive propositional calculus 8 An intuitionistically absolutely undecidable number-theoretic proposition 8–10 Quine’s theorems 181 and 180 11 Three-valued logic

34 Vol.


Page(s) 12 12–16 16–18 18–24 24–25 25–32.3 32.3–32.4 32.4–32.5 32.6–32.11 33

34–37 −54 – −52 −51 – −49.3 −49.3 – −49.2 −49.1 – −49 −49 – −47 −47 – −43 −42 – −36.1 −35 – −34 −34 – −28 −28 – −24 −24 – −23 −22 – −21 −21 – −19 −19 −18 loose page −17 – −15 −15 – −13 −12 −10 −12 – −5 −4 – −1 VIII

1–3

Translated Heading Rosser’s consistency proof Gentzen Rosser Gentzen Heyting’s functional calculus Gentzen Consistency proof for analysis ¬∼A is provable if A is classically provable Attempt at a direct proof of ¬(p) (p ∨ −p) Extension of the consistency proof to the case that quantifiers appear in the recursive definitions (and thereby to ramified type theory) Gentzen’s consistency proof Continuum hypothesis with recursive functions Attempt [to derive an] antinomy from the provability of the continuum hypothesis Axiomatic characterization of the [unreadable character] relation Rosser’s consistency proof Continuum hypothesis with recursive functions Rosser’s consistency proof von Neumann’s axioms with 2-place functions— proof Proof of von Neumann’s axioms Continuum with recursive functions Relation calculus Continuation of continuum [with] recursive [functions] Insertion on the relation calculus Continuum hypothesis with recursive functions Rosser’s consistency proof 4-valued logic and Lewis’ calculus Interpretation of 3-valued logic in Lewis’ caluclus 3-valued logic interpreted in Lewis’ calculus ¨ Schonfinkel’s Ann[als?] work Consistency [of ] Quine[’s system] Insertion on the consistency of set theory Which ordinal numbers are definable in the system of analysis Failed attempt to find a contradiction in analysis [Missing]


Vol.

Page(s) 4 5–12 13–15 16–22 22–23 23–25 26–30 30–32

35

Translated Heading Program [of his work in progress or planned] [Unlabelled] Definability of ordinal numbers and ordinal-number functions The first ordinal not constructible in P1 (P2 ) Reduction of number-theoretic proofs by induction up to 0 to inductive definitions up to 0 The first ordinal undefinable in P1 , according to Veblen’s method of continuous functions Connection between the finitary system of finite types and 0 Program goals

IX

(Intuitionistic mathematics) 1 [Unlabelled list] 2–3 Lecture, summer 1941 4 [Blank] 5–17 Gentzen’s proof (interpretation of intuitionistic logic) 18–25 Complete induction for schemes of ramification (definition and proof) 25– 41 Connection between 0 and the system of finite types 42–63 Intuitionistic logic 64–72 Intuitionistic logic of assumptions 73–74 Equality rule 74–81 Intuitionistic logic (without the assumption rule [deduction rule?]) 82–85 Intuitionistic logic of assumptions 85–89 Proof that A = ¬(f) [(x) f(x) = 0 ∨ (∃x) f(x) = 0] is consistent

X

1–4 [Missing] 5–9 Intuitionistic logic 10–21 [Missing] 22–25 Connection between 0 and the system of finite types 26–29 Reduction of schemas of inductive definition in Σ to exponentiation, and elimination of impredicative definitions 30–33 Connection between 0 and the system of finite types 33–36 Proof of the axiom of extensionality for Σ 36–38 Connection between 0 and the system of finite types 38 Finitary recursion for functions on ordinal numbers

36 Vol.

XI


Page(s) Translated Heading 39– 41 Connection between 0 etc. by means of finitary functions (schemes of ramification) 42– 43 Proof of the axiom of extensionality in Σ 44–58 Proof of the consistency of ¬(p) (p ∨ ¬p) 1–24 25–27 28–33 34–37 37–41 41– 42 43– 46 46– 47 47–54 55–56 56–57 58

XII

Consistency of ¬(p) (p ∨ ¬p) [Blank] Principal lemma of Gentzen’s consistency proof with choice sequences Consistency of analysis Uniqueness of quaternion fields Metrics in spaces not restricted [to] functions on the whole [real] line Lowest-type computable functions Metrics in spaces not restricted [to] functions on non-comp[act?] sets Souslin’s problem Principal lemma, Herbrand-Gentzen consistency proof Constraints on the independence of the continuum hyp[othesis] Germs of orderings

1 [Page references] 2 [Blank] 3–12 Brouwer, Academy of Sciences, Amsterdam [reference to one of Brouwer’s articles?] 13–16 Germs of orderings 16–19 Proof of lemma on p. 58 of workbook 11 19–21 Herbrand’s axioms for the functional calculus 22–26 Abstract content of the Herbrand/Gentzen consistency proof 26–28 Metric [on] continuous functions in [Baire’s] null space 28–35 Computable choice-sequence functions of first type 36–45 Heyting’s and Lewis’ propositional calculus 46 Mathematics and philosophy 46–49 Brouwer’s interpretation of the quantifiers 49–59 Computable functions 60–62 Brouwer’s interpretation of analysis 62–65 The axiom of extensionality in Brouwer’s interpretation


37

Vol.

Page(s) Translated Heading 65–74 Computable functions 75–100 Brouwer’s interpretation of analysis

XIII

i [Page references] 1–6 Brouwer’s interpretation of analysis 7–9 Insertion: measurability of the function which actually guarantees a well-ordering 9–29 Brouwer’s interpretation of analysis 30–31 Program [of work planned or in progress] 31–39 Functions of the classes F , G 40–42 Brouwer’s interpretation of analysis 42– 44 Extendability of elements in schemes of ramification 44–46 Regular sequences of ordinal numbers 46–51 Extension of measures 51 Undecidable propositions in systems of ordinals 52–54 Program [of work planned or in progress] 54–56 Problems and questions 56–58 Construct[ive] model of analysis according to my first method 59–end Theory of computable functions of arbitrary type

XIV

1–13 14–20 21–23 24–34 34–43 44–49 50–54 55–57 58–93

XV

1 1–2 3–7

[Continuation of workbook 13] Independence of the axiom of choice Insertion: Brouwer Independence of the axiom of choice Zermelo set theory Fregean solution of the antinomies Computable functions of integers Solution of the antinomies without type theory Consistency of the axiom of replacement

[Index] [Unlabelled] Existence of models for consistent B-systems (wrong) 8–11 Consistency of the axiom of choice according to the new method 11–13 Relative consistency of the axiom of extensionality 13–16 New axiom system for all B-tautologies 16–19 Omission of B(A) ⊃ A and replacement by iteration of the B-symbol (wrong) 19–25 Generalization of the unprovability of consistency, with few assumptions (wrong)

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Vol.

Page(s) Translated Heading 26–28 Independence of the axiom of choice 28–38 General theory of B theories about objects that form a topological space 38– 41 Application to the independence of the axiom of choice 41– 42 Attempt to extend the above to functions (instead of relations) 42–44 Theory of the B symbol under the assumption that the truth of the propositions considered depends on the wording of the definition, the objects occurring therein, and a “world” given by a point of a topological space 44– 48 The axiom of complete induction and the descriptive operator in this theory 48–49 The axiom of choice 49–51 Modification of the earlier definition of Wt (f(g)) and of the type-hierarchy 51 Possible reasonable extension of the B-axioms (insertion) 52 Which notions of truth (not falsity) are suitable for consistency proofs? 53–96 Conjecture about a property of the notion of well ordering in the B-theory

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i [Index] 1–8 [Unlabelled] 9–10 On the work [his own essay] about Russell’s logic 10–23 Ramified type theory (attempt at a consistency proof) 23–26 Aristotelean theory of modal inferences 26–27 Simpson’s Rule 27–30 Euler’s summation formula 31–32 Quantum mechanics (one-body system) 33–38 General relativity theory [is] perhaps classically interpretable 38–41 Independence of the axiom of choice (consistency of the existence of lawless sets) 42– 44 Metrization of the space of G -[sets] (respectively, function of the second class) so that monotone (respectively, functional) convergence implies metric convergence (or conversely)


Vol.

39

Page(s) Translated Heading 44–48 Non-existence of certain “regular” assignments of αi → α for each α < Ω 49 Difference sequences from n!, and similar [things] 50 Problems of abstract set theory 50–53 Absolute definability 54 Principle for constructing axioms of infinity (Mahlo) 55–59 Theta functions 60–61 Error theory for large angles 62–71 Correction due to subdivision into only 6 intervals 72–81 Bessel’s differential equation (solution by quadrature) 80–85 Impossibility of an absolute (i.e., reflexive) proof concept within a finite language, and similar [things] 86–89 Idea of the proof

Appendix C: Translated Headings in Gödel’s “Results on Foundations” Notebooks. Vol.

Pages

Topic

I

1–35 [Unlabelled lemmas and remarks] 36–37 Intensional antinomies 37–38 The following axioms are contradictory 39 Other axioms from which a contradiction likewise follows 40–42 Complete axiom system for ∼ [and] · 42–45 Another complete axiom system for ∼ [and] · 45–end Some theorems about definability of sets insert Class calculus

II

53–66 66–153

III

154–187 188–191 191–193 193–196 196–204 204–215 216–222 223–225 225–241

[Unlabelled] Positive propositional calculus Continuation of volume II Every function of the strictly intuitionistic system is computable (January 1, 1941) Theorem about the lengths of proofs Remarks about quaternions [Unlabelled (logic)] Principal theorem of Gentzen’s consistency proof Souslin’s problem and questions dependent on it Undecidable propositions according to Rosser Equivalence of Heyting’s and Lewis’ calculi

40 Vol.


Pages 242–248 249–254 254–285 258–262 263–267

IV

268–270 270–276 277–283 283–287 287–289 289–294 295 296–297 297–303 304–311 311–314 315–319 320–322 322–353 354–356 356–359 360–363 364–365 365–366 366–end

Topic Topology and Lewis’ calculus Continuation of p. 241 Existence of infinitely many truth values in Heyting’s and Lewis’ calculi Monotone sequences of F [sets] of type ≥ 1 Decomposition of complete graphs of power ℵ0 “Distinguished” enumerations of the ordinal numbers of the second class Decomposition of complete graphs of power ℵ1 Maximal well-ordered subsets in ordered sets ¨ Konig’s Theorem for arbitrary cardinalities Countable fields of sets without absolutely additive measures Extension of finitely additive measures Fixed points of monotone set functions Analytic and recursively enumerable sets Interpretation of Heyting[’s calculus] with quantifiers in Lewis’ [calculus] Sufficient properties of a model for the B-calculus so that intuitionistic logic holds (in my interpretation) Axiomatization of the B-calculus without the Badjunction rule Type-free logic is contradictory even if a substitution operation is used instead of The notion “definable” in the absolute sense Results and problems surrounding the continuum hypothesis Remarks about perturbation theory Results on foundations Consistency of analysis Cosmological problems Continuum hypothesis Remarks about the consistency of analysis

Appendix D: Logic and Foundations Notebooks: Translated Index Entries. Entries that are merely citations of works of others are not listed. Topic

Pages

My own theory of cofinality

61–70


Topic Properties of the first inaccessible cardinal and unsolvability of the measure problem and questions dependent upon it Analytic and Borel sets Questions and problems, etc. Considerations about Heyting’s propositional calculus Formalization of finitary mathematics [using] higher types, and the ordinal numbers in it [Unlabelled notes] Model for the finitary system of finite types in classical number theory Ordinals in classical number theory Characterization of the numbers < 0 Two remarks about the 2nd number class Formalization of 0 in finitary systems of finite types

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Pages 78–88

93–109 113–116 117–134 135–147 147–148

149–150 151–156 156 157–162, 165–170, 173–177 Abortive attempt to reconstruct Gentzen’s proof in 135–147, 162–164 a finitary system General[ized] functional calculus (Church) 171–173 ¨ Schroder-Bernstein [done] constructively 181 My own remarks about the subdivision of the foun- 219–220 dations [of mathematics] [Crossed-out material] 221 Propositional calculus from one axiom 267–271 Error in Principia Mathematica 273 Exclusive “or” 274 Positive propositional calculus 295–302, 333–334, 336–348, 361–363 Equivalence of different definitions of a topological 323–325 space Theorem about the equivalence of the two defini- 326–328 tions of “bicompact” Pairs in Church’s systems 354–360 Baire’s example of sets of the 3rd class 364–367 Lusin’s theorem on monotone sequences of F [sets] 368–373 Kuratowski’s and Hausdorff’s proofs for sequences 374–381 of F G [sets]

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Topic Foundations of the theory of Baire functions (incomplete) Foundations of the theory of lattices (lecture by von Neumann, 1942) Symbolism [of ] Principia

Pages 390–395 395–432 432–439

Note: The 6 volumes are consecutively numbered. Entries are not dated, but “Begin America” is written on p. 282 of vol. 5. REFERENCES

[1] John W. Dawson, Jr., Logical dilemmas: The life and work of Kurt Gödel, AK Peters, Wellesley, Mass., 1997. [2] Solomon Feferman, The Gödel editorial project: A synopsis, The Bulletin of Symbolic Logic, vol. 11 (2005), no. 2, pp. 132–149, also reprinted in this volume. [3] Kurt Godel, Collected works, vol. I: Publications, 1929–1936 (Solomon Feferman et al., ¨ editors), Oxford University Press, New York and Oxford, 1986. [4] , Collected works, vol. II: Publications, 1938–1974 (Solomon Feferman et al., editors), Oxford University Press, New York and Oxford, 1990. , Collected works, vol. III: Unpublished Essays and Lectures (Solomon Feferman [5] et al., editors), Oxford University Press, New York and Oxford, 1995. [6] , Collected works, vol. IV: Correspondence A—G (Solomon Feferman et al., editors), Oxford University Press, Oxford, 2003. [7] , Collected works, vol. V: Correspondence H—Z (Solomon Feferman et al., editors), Oxford University Press, Oxford, 2003. [8] Paolo Mancosu, Between Vienna and Berlin: The immediate reception of Gödel’s incompleteness theorems, History and Philosophy of Logic, vol. 20 (1999), no. 1, pp. 33– 46. [9] , On the constructivity of proofs. A debate among Behmann, Bernays, Gödel and Kaufmann, Reflections on the foundations of mathematics (Wilfried Sieg, Richard Sommer, and Carolyn Talcott, editors), AK Peters, Natick, Mass., 2002, pp. 349–371. [10] Maria-Elena Schimanovich-Galidescu, Archivmaterial zu Kurt Gödels Wiener Zeit 1924–1940, Kurt Gödel: Wahrheit & Beweisbarkeit, Bd. I: Dokumente und historische Analysen ¨ ¨ (Eckehart Kohler et al., editors), Holder-Pichler-Tempsky, Vienna, 2002, pp. 135–147. 393 WATERS ROAD YORK, PA 17403, USA

E-mail: [email protected] and [email protected]

PROOF THEORY

¨ GODEL AND THE METAMATHEMATICAL TRADITION†

JEREMY AVIGAD

Abstract. The metamathematical tradition that developed from Hilbert’s program is based on syntactic characterizations of mathematics and the use of explicit, finitary methods in the ¨ metatheory. Although Godel’s work in logic fits squarely in that tradition, one often finds him curiously at odds with the associated methodological orientation. This essay explores that tension and what lies behind it.

§1. Introduction. While I am honored to have been asked to deliver a lec¨ ture in honor of the Kurt Godel centennial, I agreed to do so with some hesitations. For one thing, I am not a historian, so if you are expecting late¨ breaking revelations from the Godel Nachlass you will be disappointed. A ¨ more pressing concern is that I am a poor representative of Godel’s views. As a proof theorist by training and disposition, I take myself to be working in the metamathematical tradition that emerged from Hilbert’s program; while ¨ I will point out, in this essay, that Godel’s work in logic falls squarely in this ¨ tradition, one often senses in Godel a dissatisfaction with that methodolog¨ ical orientation that makes me uneasy. This is by no means to deny Godel’s significance; von Neumann once characterized him as the most important logician since Aristotle, and I will not dispute that characterization here. But admiration does not always translate to a sense of affinity, and I sometimes ¨ have a hard time identifying with Godel’s outlook. ¨ I decided to take the invitation to speak about Godel as an opportunity to work through this ambivalence by reading and thinking about his work. This essay is largely a report on the outcome. In more objective terms, my goal will be to characterize the metamathematical tradition that originated ¨ with Hilbert’s program, and explore some of the ways that Godel shaped and reacted to that tradition. But I hope you will forgive me for adopting † This essay is only slightly modified from the text of a lecture presented at the spring meeting ¨ of the Association for Symbolic Logic in Montreal in May, 2006. Citations to Godel refer to his Collected Works [9], where extensive editorial notes and full bibliographic details can be found. I am grateful for comments, corrections, and suggestions from Mark van Atten, Solomon Feferman, Neil Tennant, Richard Zach, and a number of people at the meeting.

Kurt Gödel: Essays for his Centennial Edited by Solomon Feferman, Charles Parsons, and Stephen G. Simpson Lecture Notes in Logic, 33 c 2010, Association for Symbolic Logic

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¨ a personal tone; what I am really doing is discussing aspects of Godel’s work that are of interest to me, as a working logician, in the hope that you will find them interesting too. ¨ Godel’s work can be divided into four categories:1 • Early metamathematical work – The completeness and compactness theorems for first-order logic (1929) – The incompleteness theorems (1931) – Decidability and undecidability for restricted fragments of firstorder logic (1932, 1933) – Properties of intuitionistic logic, and the double-negation translation (1932, 1933) – The provability interpretation of intuitionistic logic (1933) – The Dialectica interpretation (1941/1958) • Set theory – The relative consistency of the axiom of choice and the continuum hypothesis (1938) • Foundations and philosophy of physics – Rotating models of the field equations (1949) • Philosophy of mathematics (ongoing) ¨ Akihiro Kanamori and Sy Friedman discuss Godel’s work in set theory in their contributions to this volume, and the contributions by Steve Awodey, ¨ John Burgess, and William Tait discuss philosophical aspects of Godel’s work. I am in no position to discuss his work on the foundations of physics, and so I will focus here almost exclusively on the first group of results. The outline of this essay is as follows. In Section 2, I will characterize what I take to be the core methodological components of the metamathematical ¨ tradition that stems from Hilbert’s program. In Section 3, I will survey Godel’s work in logic from this perspective. In Section 4, I will digress from my ¨ narrative to discuss Godel’s proof of the completeness theorem, since it is a lovely proof, and one that is, unfortunately, not well known today. In Section 5, ¨ I will consider a number of Godel’s remarks that show him to be curiously at odds with the metamathematical tradition in which he played such a central role. Finally, in Section 6, I will describe the attitude that I take to lie behind these critical remarks, and argue that recognizing this attitude is important to ¨ appreciating Godel’s contributions.2 1 The dates indicated generally correspond to the first relevant publication. For a more detailed ¨ ¨ overview of Godel’s work, see Feferman’s introduction to Godel’s Collected Works, [9, volume I]. 2 In his essay, Kurt G¨ odel: Conviction and caution [7], Feferman addresses the closely related ¨ issue of the relationship between Godel’s use of formal methods and his objectivist, or platonist, views of mathematics. There, he assesses some of the same data that I consider in Section 5. Although his analysis and conclusions differ from mine, our views are not incompatible, and ¨ provide complementary perspectives on Godel’s later remarks.

¨ GODEL AND THE METAMATHEMATICAL TRADITION

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§2. Hilbert’s program and metamathematics. Although Hilbert’s program, in its mature formulation, did not appear until 1922, Hilbert’s interest in logic and foundational issues began much earlier. His landmark Grundlagen der Geometrie of 1899 provided not just an informal axiomatic basis for Euclidean geometry, but also an extensive metamathematical study of interpretations of the axioms. (He used this, for example, to prove their independence.) A year later, he presented his famous list of twenty-three problems to the Second International Congress of Mathematicians. Three of these had a distinctly foundational character, having to do with Cantor’s continuum problem, the consistency of arithmetic, and a mathematical treatment of the axioms of physics. In 1904, he presented a partial and flawed attempt to treat the consistency problem in syntactic terms. He did not publicly address foundational issues again until 1922, save for a talk on axiomatic thought in 1917; but lecture notes and other evidence show that he was actively engaged in the issues for much of the intervening period.3 By 1922, the Grundlagenstreit resulting from Brouwer’s intuitionistic challenge was gathering steam. It was then that Hilbert presented his program to secure the methods of modern mathematics, and hence to “settle the problem of foundations once and for all.” The general features of the program are by now well known: one was to characterize the methods of modern, infinitary reasoning using formal axiomatic systems, and then prove those systems consistent, using secure, “finitary” methods. This program is often taken to presuppose a “formalist” position, whereby mathematics is taken to be nothing more than a game of symbols, with no meaning beyond that given by the prescribed rules. One finds such characterizations of formalism, for example, in Brouwer’s inaugural address to the University of Amsterdam as early as 1912 [4], and in Ramsey’s “Mathematical Logic” of 1926 [12]. When Hilbert is emphasizing the syntactic nature of his program, his language sometimes suggests such a view, but I think it is silly to take this position to characterize his attitudes towards mathematics in general. When one ignores the rhetoric and puts the remarks in the relevant context, one is left with two simple observations: first, with a syntactic characterization of infinitary mathematical reasoning in hand, the question of consistency becomes a purely mathematical question; and, second, a consistency proof using a restricted, trusted body of methods would provide solid reassurance to anyone concerned that infinitary methods might be unsound. Thus, I take the core methodological orientation of Hilbert’s program to be embodied in the following claims: • Formal axiomatic systems provide faithful representations of mathematical argumentation. 3 Bibliographic data on the works mentioned here can be found in any of [6, 9, 17]. For more on Hilbert’s program, see [1, 11, 14, 21].

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• With these representations, at least some foundational and epistemological questions can be formulated in mathematical terms. • A finitary, syntactic perspective makes it possible to address such questions without presupposing substantial portions of the body of mathematics under investigation. In particular, the formal axiomatic method makes it possible to use mathematical methods to address the question as to the consistency of infinitary reasoning, without presupposing the existence of infinitary objects in the analysis. I, personally, subscribe to these views, and find them eminently reasonable. Taken together, they allow one to use mathematical methods to address epistemological questions, resulting in clear and concrete philosophical gains. Since Hilbert’s day, there has been an explosion of interest in computational and symbolic methods in the sciences, while, at the same time, important branches of mathematics have developed methods that are increasingly abstract and removed from computational interpretation. For that reason, I take this broader construal of Hilbert’s program to be as important today as it was in Hilbert’s time, if not even more so. §3. Gödel and the metamathematical tradition. With this characterization ¨ of the metamathematical tradition in hand, let us now turn to Godel’s work ¨ in logic. In his 1929 doctoral dissertation, Godel proved the completeness theorem for first-order logic, clarifying a relationship between semantic and syntactic notions of logical consequence that had bedeviled early logicians [3]. The dissertation makes frequent references to Hilbert and Ackermann’s 1928 Grundzüge der theoretischen Logik, where the problem of proving completeness for first-order logic was articulated clearly. The compactness theorem, ¨ which was ancillary to Godel’s proof, is undeniably model theory’s most important tool. In contrast to the completeness theorem, the incompleteness theorems of 1930 are negative results in Hilbert’s metamathematical program. The first incompleteness theorem shows the impossibility of obtaining a complete axiomatization of arithmetic, contrary to what Hilbert had proposed in 1929 [10]. Of course, the second incompleteness theorem, which shows that no reasonable theory of mathematics can prove its own consistency, was a much bigger blow, since it indicates that the central goal of Hilbert’s metamathematical program cannot be attained. ¨ Over the next few years, Godel issued a remarkable stream of striking and seminal results. His double-negation interpretation of classical arithmetic (as well as classical logic) in intuitionistic arithmetic (resp. intuitionistic logic), discovered independently by Gerhard Gentzen, clarified the relationship between the two forms of mathematical reasoning that had been the subject


49

of intense discussion. His interpretation of intuitionistic propositional logic in a modal logic with a “provability” operator helped clarify the relationship between provability and the intuitionistic connectives. His results on the decidability and undecidability of various fragments of first-order logic are fundamental, and are close to being optimal and exhaustive for the first-order setting. ¨ Although Godel’s Dialectica interpretation of arithmetic was not published until 1958, he obtained the results much earlier, and lectured on them at Yale in 1941. The interpretation amounts to a translation of intuitionistic arithmetic (and, via the double-negation translation, classical arithmetic) in a quantifier-free theory of primitive recursive functionals of higher-type. This reduces induction for formulas that quantify over the infinite domain of natural numbers to explicit, quantifier-free, induction, modulo a computational scheme of primitive recursion in the higher types. ¨ It is worth mentioning that Godel’s contributions to the study of computability are not only fundamental to computer science today, but firmly in the tradition of the Hilbert school. These include his work on formal notions of computability, which was an important by-product of his work on incompleteness, and the study of primitive recursion in the higher types that accompanies the Dialectica interpretation. ¨ While reading up on Godel for this essay, I was struck by a remarkable fact: ¨ all of Godel’s results in logic, except the completeness theorem, are syntactic in nature.4 That is to say, every theorem has to do with either provability in a formal system, a translation between formal systems, or the existence of an algorithm for determining whether or not something is provable. Moreover, all the proofs, except for the proof of the completeness theorem, are explicitly finitary, and can be formalized straightforwardly in primitive recursive arithmetic. This is true even of his work in set theory, as he is careful to point out in every statement of the results. For example, in his abstract announcing the relative consistency of the axiom of choice and the continuum hypothesis in 1938, he emphasizes: The proof of the above theorems is constructive in the sense that, if a contradiction were obtained in the enlarged system, a contradiction ¨ 1938, II, p. 26) in T could actually be exhibited.5 (Godel The exception is the proof of the completeness theorem, which, of course, is nonconstructive. The introduction to his dissertation closes with the following 4 There is another small exception, namely, a short note on the satisfiability of uncountable ¨ sets of sentences in the propositional calculus (Godel 1932c, I, pp. 238–241). 5 In other words, we have a finitary proof that if set theory without the additional principles is consistent, then it remains so when the new principles are added as axioms. The double-negation interpretation of classical arithmetic in intuitionistic arithmetic has a similar character; see, for ¨ example, the last paragraph of (Godel 1933e, I, p. 295).

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comments: In conclusion, let me make a remark about the means of proof used in what follows. Concerning them, no restriction whatsoever has been made. In particular, essential use is made of the principle of the excluded middle for infinite collections (the nondenumerable infinite, however, is not used in the main proof). It might perhaps appear that this would invalidate the entire completeness proof. ¨ (Godel 1929, I, p. 63) What follows this passage is a discussion of the relevant epistemological issues, and the sense in which the completeness theorem is informative. I do not want ¨ to go into the anticipated criticism of the results, or Godel’s response. Rather, ¨ I wish to highlight Godel’s remarkable sensitivity to the question as to what metamathematical methods are necessary to obtain the requisite results, and the impact these methods have on the epistemological consequences. ¨ Godel’s proof is not often presented these days, which is a shame, because it is interesting and informative. I will therefore break from my narrative, briefly, to share it with you now. §4. Gödel’s proof of the completeness theorem. I will take some liberties in describing the proof. For example, I will use contemporary terminology and notation throughout, and rearrange the order in which some of the ideas are presented. A historian will be able to point out all the ways in which my modern gloss ignores interesting and important historical nuances.6 But one ¨ does not have to be a historian to read and enjoy Godel’s original article; what is striking is the extent to which such a young researcher in a new subject could produce such a clear and mature presentation. 6 One issue that I have set aside is the influence of Skolem’s work on Godel. ¨ In papers published ¨ in 1920 and 1923, Skolem gave two clarified and improved proofs of Lowenheim’s 1915 theorem, ¨ both of which are reprinted in [17]. The normal form used by Godel below is taken from ¨ Skolem’s 1920 paper, which is acknowledged in Godel’s 1929 dissertation and in the version of the proof published in 1930. In fact, if one replaces satisfiability by consistency in Skolem’s ¨ ¨ 1923 paper, the result is essentially Godel’s proof. Godel later acknowledged this fact (this is the context of the quote on the “blindness of logicians” in Section 5, below), but claimed he did not know of Skolem’s 1923 proof when he wrote the dissertation. Mark van Atten [15] has combed ¨ ¨ through the Godel Nachlass and has discovered library slips showing that Godel requested the volume with Skolem’s paper on three separate occasions, but each time the library reported that it was unable to secure the volume. ¨ Another interesting issue has to do with the use of what we now call Konig’s lemma, which ¨ ¨ was used in papers by Konig in 1926 and 1927. Godel seems to be unaware of this, since he does ¨ ¨ not cite Konig in either the dissertation or the paper. Godel gave a quick proof of the lemma in the dissertation, and in the paper said only that the desired truth assignment can be obtained “by familiar arguments.” It is worth noting that Skolem also provided a proof of the lemma in his 1923 paper. These issues are well covered in Dreben and van Heijenoort’s introductory notes to ¨ (Godel 1929, 1930, and 1930a) in volume I of the Collected Works.


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¨ Godel states the completeness theorem in the following form: “if a firstorder sentence ϕ is not refutable, then it has a model.” He also considers the stronger, infinitary version, where ϕ is replaced by a set of sentences, Γ. He restricts attention to countable first-order languages, so Γ is at worst countably infinite. Step 1. If a propositional formula ϕ is not refutable, it has a satisfying truth assignment. This was proved by Post and Bernays, independently, around 1918. One way to prove it is to simply simulate the method of checking each line of the truth table, in the relevant deductive system. Step 2. If a set Γ of propositional formulas is not refutable, it has a satisfying truth assignment. Write Γ = {ϕ0 , ϕ1 , ϕ2 , . . . }. Build a finitely branching tree where the nodes at level one are all the truth assignments to variables of ϕ0 that make ϕ0 true; the nodes at level two are all the truth assignments to variables of ϕ0 ∧ ϕ1 that make that formula true; and so on. (The descendants of a node are all the truth assignments that extend it.) If, at some level k, there is no satisfying assignment to ϕ0 ∧ ϕ1 ∧ · · · ∧ ϕk−1 , then, by step 1, Γ ¨ is refutable. Otherwise, by Konig’s lemma, there is a path through the tree, which corresponds to a satisfying truth assignment for Γ. Step 3. Now consider a first-order sentence ϕ of the form ∀x¯ ∃y¯ (x, ¯ y), ¯ where is quantifier-free in a language with neither function symbols nor the equality symbol. We can prove that ϕ is either refutable or has a model, as follows. Add countably many fresh constants to the language, let c¯ i denote an enumeration of all the tuples of constants that can be substituted for x, ¯ and define a sequence of sentences i recursively by taking i ≡ (c¯ i , ck , ck+1 , . . . , ck+l ) where ck , ck+1 , . . . , ck+l do not appear in 0 , . . . , i−1 . The idea is that we are trying to build a model of ϕ whose universe consists of the constant symbols, so at each stage i we choose new constants to witness the truth of ¯ Now treat the atomic sentences in the language, which are of the ∃y¯ (c¯ i , y). form R(ci0 , . . . , cim ), as propositional variables. By step 2, either some finite subset of {i | i ∈ N} is propositionally refutable, or there is a satisfying truth assignment. In the second case, we get a model of ϕ by taking the universe to be the set of constant symbols and using the truth assignment to determine which relations hold of which tuples. In the first case, ϕ is refutable: from a refutation of Δ ∪ {(c¯ i , ck , ck+1 , . . . , ck+l )}, where ck+1 , . . . , ck+l do not occur in the finite set Δ, it is easy to obtain a refutation of Δ ∪ {∀x¯ ∃y¯ (x, ¯ y)}, ¯

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and this move can be iterated until all the formulas in Δ are replaced by ∀x¯ ∃y¯ (x, ¯ y). ¯ Note that the same method works for infinite sets of ∀∃ sentences, with only slightly more elaborate bookkeeping. Step 4. Now let ϕ be an arbitrary first-order sentence in a language without equality or function symbols. The idea is that ϕ is “equivalent” to a sentence ¯ y) ¯ is ∃R¯ ϕ , where ϕ is ∀∃. For example, a formula of the form ∀x¯ ∃y¯ α(x, “equivalent” to (1)

∃R (∀x¯ ∃y¯ R(x, ¯ y) ¯ ∧ ∀x¯ ∀y¯ (R(x, ¯ y) ¯ → α(x, ¯ y))). ¯

To see this, note that formula (1) clearly implies ∀x¯ ∃y¯ α(x, ¯ y), ¯ and the converse is obtained by taking R(x, ¯ y) ¯ to be α(x, ¯ y). ¯ But now note that if α is a prenex formula with k + 1 ∀∃-blocks of quantifiers, the formula after ∃R in (1) can be put in prenex form, with only k such blocks. Iterating this, we end up with a formula in the desired normal form. More precisely, the sense in which ϕ is equivalent to ∃R¯ ϕ is this: • ϕ → ϕ is provable in first-order logic, so that if ϕ has a model, so does ϕ; and • if ϕ is refutable, then so is ϕ. So the statement “if ϕ is not refutable, then it has a model” is reduced to the corresponding statement for the ∀∃ formula ϕ , and we have already handled that case in Step 3. Once again, the proof extends straightforwardly to infinite sets of sentences. Step 5. The result can be extended to languages with function symbols, by interpreting functions in terms of relations; and to languages with equality, in the usual way, by adding equality axioms and then taking a quotient structure. What do I like about the proof ? First, it is extremely modular. Each step turns on one key idea, and the proof clearly dictates the requisite properties of the deductive system: the role of the propositional axioms and rules is clear in Step 1; the quantifier rules and axioms are used in Steps 3 and 4; the equality axioms only come in at Step 5. Second, the constructive content is clear; the only nonconstructive element ¨ is the use of Konig’s lemma in Step 2. This observation lies at the heart of recursion-theoretic and proof-theoretic analyses of the completeness theorem. Third, the proof shows something stronger: if a formula ϕ does not have a model, there is a refutation of ϕ that involves only propositional combinations of subformulas of ϕ. In particular, if a formula is provable in first-order logic (and so ¬ does not have a model), then there is a proof of involving only formulas with a quantifier complexity that is roughly the same as that of , or lower. This is an important proof-theoretic fact that is usually obtained as


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a consequence of the cut-elimination theorem, and so I was surprised to find ¨ it implicit in Godel’s original proof. Finally, there is the choice of normal form. Skolem functions can be used to reduce the satisfiability of a sentence to the satisfiability of a universal sentence, its Skolem normal form. This fact is used often in proof theory and automated reasoning today. But there are messy technical difficulties involved with eliminating Skolem axioms; since Skolem functions are really choice functions, this is closely related to mathematicians’ dislike of noncanonical ¨ choices in an ordinary mathematical proof. Godel uses the fact that the satisfiability of any first-order formula can be reduced to the satisfiability of an ∃∀ sentence in a purely relational language. The quantified relations are really choice-free “Skolem multifunctions,” making the reduction technically smoother and much more satisfying. §5. Gödel’s remarks on finitism. I would like to return to the relationship ¨ between Godel and Hilbert, and, by way of contrast, briefly consider the relationship between Hilbert and his predecessor, Kronecker. Hilbert’s early work in algebraic geometry and algebraic number theory was strongly influenced by that of Kronecker, though, as is well-known, Hilbert was critical of Kronecker’s methodological proscriptions for mathematics. Indeed, Hilbert’s program can be seen as an attempt to do battle with Kronecker, on Kronecker’s own terms. In his obituary for Hilbert, Weyl colorfully described the situation as follows: When one inquires into the dominant influences acting upon Hilbert in his formative years one is puzzled by the peculiarly ambivalent character of his relationship to Kronecker: dependent on him, he rebels against him. Kronecker’s work is undoubtedly of paramount importance for Hilbert in his algebraic period. But the old gentleman in Berlin, so it seemed to Hilbert, used his power and authority to stretch mathematics upon the Procrustean bed of arbitrary philosophical principles and to suppress such developments as did not conform: Kronecker insisted that existence theorems should be proved by explicit construction, in terms of integers, while Hilbert was an early champion of Georg Cantor’s general set-theoretic ideas. . . . A late echo of this old feud is the polemic against Brouwer’s intuitionism with which the sexagenar¨ ian Hilbert opens his first article on “Neubegrundung der Mathematik” (1922): Hilbert’s slashing blows are aimed at Kronecker’s ghost whom he sees rising from the grave. But inescapable ambivalence even here — while he fights him, he follows him: reasoning along strictly intuitionistic lines is found necessary by him to safeguard non-intuitionistic mathematics. (Weyl [20], p. 613)

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¨ The relationship between Godel and Hilbert is not nearly as dramatic. ¨ I have characterized Godel’s work as being firmly in the tradition that Hilbert established, much of it devoted to answering questions that Hilbert himself ¨ posed. In that regard, Godel gives credit where it is due, and does not in any way deny Hilbert’s influence or play down the importance of his contributions. In fact, his 1931 paper on the incompleteness theorems ends with a remarkably charitable and optimistic assessment of Hilbert’s program: I wish to note expressly that [the statements of the second incompleteness theorem for the formal systems under consideration] do not contradict Hilbert’s formalistic viewpoint. For this viewpoint presupposes only the existence of a consistency proof in which nothing but finitary means of proof is used, and it is conceivable that there exist finitary proofs that cannot be expressed [in the relevant ¨ formalisms]. (Godel 1931, I, p. 195) Within a few years, however, he had abandoned this view.7 In his lecture at Zilsel’s seminar in 1938, he was much more critical of attempts to salvage Hilbert’s original plan to establish the consistency of mathematics. Commenting on Gentzen’s proof of the consistency of arithmetic using transfinite induction up to ε0 , he says: I would like to remark by the way that Gentzen sought to give a “proof” of this rule of inference and even said that this was the essential part of his consistency proof. In reality, it’s not a matter of proof at all, but of an appeal to evidence. . . I think it makes more sense to formulate an axiom precisely and to say that it is just not further reducible. But here again the drive of Hilbert’s pupils ¨ to derive something from nothing stands out. (Godel 1938a, III, pp. 107–109) In later years, one finds him generally critical of a finitist methodology. For example, in a letter he wrote to Hao Wang in 1967, he blamed the failure of Skolem to extract the completeness theorem from his results of 1923 on the intellectual climate of that time, and to a misplaced commitment to a finitist metatheory: This blindness (or prejudice, or whatever you may call it) of logicians is indeed surprising. But I think the explanation is not hard to find. It lies in a widespread lack, at that time, of the required epistemological attitude towards metamathematics and toward non-finitary reasoning. 7 See,

¨ for example, Godel *1933o in [9, volume II].


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Non-finitary reasoning in mathematics was widely considered to be meaningful only to the extent to which it can be “interpreted” or “justified” in terms of a finitary metamathematics. (Note that this, for the most part, has turned out to be impossible in consequence of my results and subsequent work.) (Quoted in [18, p. 8], and [19, pp. 240–241]) ¨ Despite the fact that almost all of his proofs were explicitly finitary, Godel went out of his way to emphasize that the “objectivistic conception of mathematics and metamathematics in general, and of transfinite reasoning in particular, was fundamental to my other work in logic.” Of course, by repre¨ senting transfinite methods within explicit formal systems, Godel can make use of such reasoning while maintaining finitary significance. But it is in¨ teresting that here Godel plays up the importance of the transfinite methods, while downplaying the importance of the finitary metamathematical stance. A few months later, in a follow-up to that letter, he repeated the claim that it would have been practically impossible to discover his results without an objectivist conception. He then continued: I would like to add that there was another reason which hampered logicians in the application to metamathematics, not only of transfinite reasoning, but of mathematical reasoning in general. It consists in the fact that, largely, metamathematics was not considered as a science describing objective mathematical states of affairs, but rather as a theory of the human activity of handling symbols. [18, pp. 9–10] This last passage indicates a critical attitude towards syntactic characteriza¨ tions of mathematics that one also finds in an essay that Godel began preparing in 1953 for the Schilpp volume on Carnap. The essay was titled “Is mathematics the syntax of language?” and was designed to refute this core tenet of logical positivism. Although he never completed a version that he found satisfactory, he did feel that his refutation of Carnap’s position was decisive.8 In 1972, he said to Wang: Wittgenstein’s negative attitude towards symbolic language is a step backward. Those who, like Carnap, misuse symbolic language want to discredit mathematical logic; they want to prevent the appearance of philosophy. The whole movement of the positivists 8 Awodey and Carus have shown, however, that the argument is flawed; see [2]. See also ¨ Warren Goldfarb’s introductory notes to (Godel *1953/9, III, 324–363) for a detailed discussion ¨ of Godel’s essay.

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want to destroy philosophy; for this purpose, they need to destroy mathematical logic as a tool. [19, p. 174] Although these comments are not directed at Hilbert per se, they can be viewed as a criticism of the types of formalism that are often ascribed to ¨ Hilbert. Godel did provide a direct assessment of Hilbert’s program in a lecture that he prepared for the American Philosophical Society around 1961 but ¨ never delivered. In that lecture, Godel characterized general philosophical world-views along a spectrum, in which “skepticism, materialism, and positivism stand on one side, spiritualism, idealism, and theology on the other.” ¨ The tenor of the times, according to Godel, had led towards a general shift to the skeptical values that he had located on the left side of the spectrum. Mathematics had traditionally been a stronghold for those idealistic values on ¨ the right. But, according to Godel, the skeptical attitudes eventually reached the point where they began to influence foundational thinking in mathematics, resulting in concerns about the consistency of mathematical reasoning. Although the nihilistic consequences are very well in accord with the spirit of the time, here a reaction set in—obviously not on the part of philosophy, but rather on that of mathematics, which, by its nature, as I have already said, is very recalcitrant in the face of the Zeitgeist. And thus came into being that curious hermaphroditic thing that Hilbert’s formalism represents, which sought to do justice both to the spirit of the time and to the nature of mathematics. It consists in the following: on the one hand, in conformity with the ideas prevailing in today’s philosophy, it is acknowledged that the truth of the axioms from which mathematics starts out cannot be justified or recognized in any way, and therefore the drawing of consequences from them has meaning only in a hypothetical sense, whereby this drawing of consequences itself (in order to satisfy even further the spirit of the time) is construed as a mere game with symbols according to certain rules, likewise not [supported ¨ by] insight. (Godel 1961/?, III, p. 379) ¨ On the other hand, Godel went on to explain, Hilbert’s program was designed to justify the desired “rightward” view of mathematics, via finitary consistency proofs. The incompleteness theorems, however, show that “it is impossible to rescue the old rightward aspects of mathematics in such a manner” (ibid., p. 381). Thus a more subtle reconciliation of the leftward and rightward views is required: The correct attitude appears to me to be that the truth lies in the middle, or consists of a combination of the two conceptions. Now, in the case of mathematics, Hilbert had of course attempted just such a combination, but one obviously too primitive and tending too strongly in one direction. (ibid.)


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¨ Godel is not excessively critical of Hilbert in the lecture. But while he is respectful of Hilbert’s attempt to rescue mathematics from the destructive tendencies of materialism and skepticism, he clearly feels that Hilbert’s viewpoint is inadequate to the task at hand. (The lecture goes on to suggest that the methods of Husserl’s phenomenology provide a more promising approach, but this is not something I can go into now.9 ) ¨ §6. Conclusions. In the end, what are we to make of Godel’s critical remarks? Most of the comments we have just considered were made toward the ¨ end of Godel’s life, and I think it would be a mistake to assume that such views influenced his earlier work. But the remarks do indicate a fundamental aspect ¨ of Godel’s outlook that puts it in stark opposition to Hilbert’s and which, ¨ I believe, was constant throughout Godel’s career. ¨ The fundamental difference between Godel and Hilbert, as I see it, lies in their views on the relationship between mathematics and philosophy. Hilbert was a consummate mathematician, with an unbounded optimism and faith in the ability of mathematics to solve all problems; at the same time, he was openly disparaging of the contemporary philosophical climate, and skeptical of philosophy’s ability to settle epistemological issues on its own terms.10 Thus, from Hilbert’s perspective, progress is only possible insofar as philosophy can be absorbed into mathematics, that is, insofar as one can replace philosophical questions with properly mathematical ones. ¨ What Godel and Hilbert had in common was an unshakeable faith in ra¨ tional inquiry. But, in contrast to Hilbert, Godel was intensely sensitive to the limitations of formal methods, and deemed them insufficient, on their own, to secure our knowledge of transcendent mathematical reality. Thus, for ¨ Godel, important epistemological questions require philosophical methods that go beyond the formal mathematical ones, picking up the slack where mathematical methods fall short: The analysis of concepts is central to philosophy. Science only combines concepts and does not analyze concepts. It contributes to the analysis of concepts by being stimulating for real analysis. . . . Analysis is to arrive at what thinking is based on: the inborn intuitions. [19, p. 273] This, I take it, explains his disdain for mathematicians and philosophers who expect too much from syntactic methods. They are the ones who expect 9 See van Atten and Kennedy [16] for a detailed analysis of Godel’s ¨ interest in phenomenology, and further references. 10 Carnap was even more critical of the metaphysical turn in philosophy, as it evolved from Husserl to Heidegger; see, for example, the discussion in Friedman [8].

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“to derive something from nothing” while avoiding “the appearance of philosophy.” ¨ There is a touch of drama here. Godel had inherited a powerful metamathematical tradition from Hilbert, and he shared Hilbert’s strong desire to save mathematics from destructive skeptical attitudes. But, in the end, he concluded that an overly narrow reading of the metamathematical tradition leaves skepticism with the upper hand. Remember that Hilbert ended his ¨ ¨ Konigsberg lecture, “Naturerkennen und Logik,” with the words “wir mussen wissen, wir werden wissen.”11 At the same time, unbeknownst to Hilbert, ¨ Godel was at a conference on epistemology and the exact sciences in that very same city. It is one of the great ironies in the history of ideas that this was the ¨ conference where Godel announced the first incompleteness theorem, just a day before Hilbert gave that famous speech (see [5, 68–71]). Before I began preparing to write this essay, I would have sided with Hilbert. ¨ I take Godel’s most important and enduring contributions to lie in his mathematical work; one cannot deny that the stunning corpus of theorems that he produced extend our knowledge in profound and important ways. His philosophical views on mathematical realism and the nature of our faculties of intuition seem to me to be comparatively thin. To be sure, one can imagine that had his health been better and his life been longer, he might have produced more striking and compelling theorems to fill out the informal views. But this is exactly my point: absent the mathematical analysis, it is hard to say what these views amount to. ¨ But I have come to realize that this way of separating Godel’s mathematical work from his philosophical views is misleading. For, what is most striking ¨ about Godel’s mathematical work is the extent to which it is firmly rooted in ¨ philosophical inquiry. We never find Godel making up mathematical puzzles just for the sake of solving them, or developing a body of mathematical techniques just for the sake of doing so. Rather, he viewed mathematical logic as a sustained reflection on the nature of mathematical knowledge, providing ¨ a powerful means of addressing core epistemological issues. Godel kept his focus on fundamental questions, and had the remarkable ability to to advance our philosophical understanding with concrete and deeply satisfying answers. When one considers the history of science and philosophy in broad terms, it becomes clear that the sharp separation between the two disciplines that we ¨ see today is a recent development, and an unfortunate one. In contrast, Godel saw mathematics and philosophy as partners, rather than opponents, working 11 “We must know, we will know.” A four-minute excerpt from the speech was later broadcast by radio. The text of the excerpt and a translation by James T. Smith can be found online, together with a link to an audio recording of the broadcast:

http://math.sfsu.edu/smith/Documents/HilbertRadio/HilbertRadio.pdf The final pronouncement is also Hilbert’s epitaph; see [13].


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together in the pursuit of knowledge. This conception of logic, I believe, is ¨ Godel’s most important legacy to the metamathematical tradition, and one we should be thankful for. REFERENCES

[1] Jeremy Avigad and Erich H. Reck, “Clarifying the Nature of the Infinite”: the Development of Metamathematics and Proof Theory, Technical Report CMU-PHIL-120, Carnegie Mellon University, 2001. [2] Steve Awodey and André W. Carus, How Carnap could have replied to Gödel, Carnap Brought Home: The View from Jena (Steve Awodey and Carsten Klein, editors), Open Court, Chicago, 2004, pp. 203–223. [3] Steve Awodey and Erich H. Reck, Completeness and categoricity. I. Nineteenth-century axiomatics to twentieth-century metalogic, History and Philosophy of Logic, vol. 23 (2002), no. 1, pp. 1–30. [4] L. E. J. Brouwer, Intuitionism and formalism, An English translation by Arnold Dresden appears in the Bulletin of the American Mathematical Society, vol. 20 (1913–1914) pp. 81–96, and is reprinted in Classics of Mathematics (Ronald Callinger, editor), Prentice Hall, Englewood, 1982, pp. 734–740. [5] John W. Dawson, Jr., Logical Dilemmas: The Life and Work of Kurt Gödel, A K Peters Ltd., Wellesley, 2005. [6] William Ewald (editor), From Kant to Hilbert: A Source Book in the Foundations of Mathematics. Vol. I, II, Oxford Science Publications, The Clarendon Press Oxford University Press, New York, 1996. [7] Solomon Feferman, Kurt Gödel: conviction and caution, Philosophia Naturalis, vol. 21 (1984), no. 2-4, pp. 546–562, Reprinted in Solomon Feferman, In the Light of Logic, Oxford University Press, New York, 1998, pp. 150–164. [8] Michael Friedman, A Parting of the Ways: Carnap, Cassirer, and Heidegger, Open Court, Chicago, 2000. [9] Kurt Godel, Collected Works, Oxford University Press, Oxford, 1986–2003, Edited by ¨ Solomon Feferman, John W. Dawson, Jr., Warren Goldfarb, Charles Parsons and Wilfried Sieg, Volumes I–V. [10] David Hilbert, Probleme der Grundlegung der Mathematik, Mathematische Annalen, vol. 102 (1930), no. 1, pp. 1–9. [11] Paolo Mancosu (editor), From Brouwer to Hilbert: The Debate on the Foundations of Mathematics in the 1920s, Oxford University Press, New York, 1998. [12] Frank Plumpton Ramsey, Mathematical logic, Mathematical Gazette, vol. 13 (1926), pp. 185–194, Reprinted in F. P. Ramsey, Philosophical Papers, Cambridge University Press, Cambridge, 1990, pp. 225–244. [13] Constance Reid, Hilbert, Springer-Verlag, New York, 1970. [14] Wilfried Sieg, Hilbert’s programs: 1917–1922, The Bulletin of Symbolic Logic, vol. 5 (1999), no. 1, pp. 1– 44. [15] Mark van Atten, On Gödel’s awareness of Skolem’s Helsinki lecture, History and Philosophy of Logic, vol. 26 (2005), no. 4, pp. 321–326. [16] Mark van Atten and Juliette Kennedy, On the philosophical development of Kurt Gödel, The Bulletin of Symbolic Logic, vol. 9 (2003), no. 4, pp. 425– 476. [17] Jean van Heijenoort, From Frege to Gödel. A Source Book in Mathematical Logic, 1879–1931, Harvard University Press, Cambridge, 1967. [18] Hao Wang, From Mathematics to Philosophy, Routledge & Kegan Paul, London, 1974. [19] , A Logical Journey: From Gödel to Philosophy, MIT Press, Cambridge, 1996.

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[20] Hermann Weyl, David Hilbert and his mathematical work, Bulletin of the American Mathematical Society, vol. 50 (1944), pp. 612–654. [21] Richard Zach, Hilbert’s program, Stanford Encyclopedia of Philosophy, http://plato. stanford.edu/entries/hilbert-program/. DEPARTMENT OF PHILOSOPHY CARNEGIE MELLON UNIVERSITY PITTSBURGH, PA 15213


ONLY TWO LETTERS: ¨ THE CORRESPONDENCE BETWEEN HERBRAND AND GODEL

WILFRIED SIEG

Abstract. Two young logicians, whose work had a dramatic impact on the direction of logic, exchanged two letters in early 1931. Jacques Herbrand initiated the correspondence on 7 April ¨ and Kurt Godel responded on 25 July, just two days before Herbrand died in a mountaineering accident at La Bérarde (Isère).1 Herbrand’s letter played a significant role in the development of ¨ computability theory. Godel asserted in his 1934 Princeton Lectures and on later occasions that it suggested to him a crucial part of the definition of a general recursive function. Understanding this role in detail is of great interest as the notion is absolutely central. The full text of the letter ¨ had not been available until recently, and its content (as reported by Godel) was not in accord with Herbrand’s contemporaneous published work. Together, the letters reflect broader intellectual currents of the time: they are intimately linked to the discussion of the incompleteness theorems and their potential impact on Hilbert’s Program.

Introduction. Two important papers in mathematical logic were published in 1931, one by Jacques Herbrand in the Journal für reine und angewandte ¨ Mathematik and the other by Kurt Godel in the Monatshefte für Mathematik ¨ und Physik. At age 25, Godel was Herbrand’s elder by just two years. Their work dramatically impacted investigations in mathematical logic, but also became central for theoretical computer science as that subject evolved in the fifties and sixties. The specific techniques of resolution and unification derive from ideas in Herbrand’s work, whereas the very notion of computability in ¨ the form of general recursiveness was introduced in Godel’s work three years later, with reference to Herbrand. Herbrand’s 1931-paper established the consistency of a fragment of arithmetic by elementary meta-mathematical means. These means were chosen to be “finitist” in the spirit of Hilbert’s Program, which Herbrand was pursuing. The program aimed to secure or guarantee the internal coherence of modern mathematics. Finitist consistency proofs for formal theories were the means to that end and Herbrand’s were the most far-reaching that had been ¨ obtained at the time. Godel’s 1931-paper, in contrast, showed that sufficiently Reprinted from The Bulletin of Symbolic Logic, vol. 11, no. 2, 2005, pp. 172–184. 1 As Godel ¨ sent his letter to the Paris address of Herbrand’s parents, it is almost certain that Herbrand never read the letter. Kurt Gödel: Essays for his Centennial Edited by Solomon Feferman, Charles Parsons, and Stephen G. Simpson Lecture Notes in Logic, 33 c 2010, Association for Symbolic Logic

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strong formal theories, even for arithmetic, have two general features: they are syntactically incomplete, and they cannot prove their own consistency. The first fact is the First Incompleteness Theorem, the second the Second. The Second Theorem points to limits of Hilbert’s consistency program, whereas the First shows that the totality of arithmetic truths cannot be captured in formal theories. Related to these papers are two letters, which were exchanged between ¨ Herbrand and Godel in early 1931. The letters are linked to a wider discussion on the foundations of mathematics that involved leading mathematicians, logicians, and philosophers, for example Johann von Neumann, Hilbert’s collaborator Paul Bernays, and members of the Vienna Circle such as Rudolf Carnap. The letters throw a distinctive light on this discussion, as Herbrand ¨ and Godel focus in a very open, non-ideological way on two central issues: (i) the extent of finitist or, at the time synonymously, intuitionist methods and (ii) the effect of the incompleteness theorems on Hilbert’s Program. For ¨ contemporary readers of the letters there is a third issue: Godel remarked in his 1934 Princeton Lectures and at later occasions that Herbrand had suggested to him a central part of the definition of general recursiveness in “a private communication.” The conceptually fascinating question is, (iii) what did ¨ Herbrand really suggest, and how did his suggestion affect Godel’s definition? There is a bit of mystery surrounding this private communication. Jean van ¨ Heijenoort queried Godel in 1963 about his remark, in part because there was a ¨ discrepancy between Godel’s report on Herbrand’s suggestion and Herbrand’s ¨ published remarks on related issues. Godel responded that the suggestion had been communicated to him in a letter of 1931, and that Herbrand had made ¨ it in exactly the form in which his lecture notes presented it. But Godel was unable to find Herbrand’s letter among his papers. John Dawson discovered ¨ ¨ the letter in the Godel Nachlass in 1986, and it became clear that Godel had misremembered. It is often the case that particular documents reflect broader intellectual currents, and that the analysis of such documents reveals central aspects vividly and in novel ways. This observation certainly holds for these letters. The broader intellectual currents will be sketched in Part 1, which is entitled Immediate Context: Incompleteness. Part 2 presents Herbrand’s Issue and is followed in Part 3 by Gödel’s Response(s). Part 4 looks at the Future Impact: Computability. I try to draw an informative vignette that is illuminated by a rich past and radiates into a complex future. 1. Immediate Context: Incompleteness. The sketch of the context has the structure of concentric spheres with the letters at their center. The first sphere reflects Hilbert’s proof theory that began to be pursued in a programmatically coherent form in 1922. Consistency proofs were to be given for formal theories, in which mathematics can be developed, and the proofs were to use only

¨ THE CORRESPONDENCE BETWEEN HERBRAND AND GODEL

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finitist means. The development of proof theory was embedded in the loud foundational dispute of the 1920s between Hilbert’s “Finitism,” Brouwer’s “Intuitionism,” and the “Logicism” that had been inspired by the investigations of Frege, Russell, and Whitehead; this is the second sphere. The third sphere represents the substance of the foundational dispute and reflects the intellectual tensions between Dedekind and Kronecker, which are related to the emergence of set theory in the second half of the 19th century. This emergence, in turn, is connected to a new systematic self-understanding of mathematics and a thorough reexamination of its role in the sciences; one can correctly speak of a transformation of classical mathematics into a new subject of axiomatically formulated abstract theories. The three outermost spheres can and will remain in the background, whereas the innermost one has to be described more thoroughly. Before doing that, I want to make one additional remark. Hilbert appears in all the spheres; he defends vigorously the modern conception of mathematics and yet tries to mediate the Kronecker-Dedekind tensions by his consistency program. In lectures and publications from 1922 and 1923, he established the consistency of an elementary part of arithmetic. Ackermann and von Neumann extended this result in 1924/25 but difficulties were encountered when it was attempted to extend results further. These difficulties were first thought to be of a “technical” mathematical sort, but instead were revealed by the incompleteness theorems as “conceptual” philosophical ones. Hilbert had initiated not only proof theoretic investigations, but also broader meta-mathematical studies of logic in lectures as early as the winter term of 1917/18. One interesting result, obtained in collaboration with Bernays, was the semantic completeness of sentential logic. In his Bologna talk of 1928, Hilbert posed the semantic completeness issue for full first-order logic as one ¨ in a list of problems. Godel solved it positively in his doctoral dissertation of 1929. Another problem, also formulated by Hilbert in Bologna, concerned the syntactic completeness of first-order arithmetic and Hilbert expected a ¨ positive result here as well. However, Godel obtained a negative result in the summer of 1930. He reported it in late August to friends in Vienna and a couple of weeks later, briefly and understatedly, during a roundtable discussion at ¨ ¨ a conference in Konigsberg. That’s where Godel’s and Herbrand’s paths were indirectly linked. Herbrand, too, was deeply influenced by Hilbert’s foundational enterprise and wrote a thesis entitled Recherches sur la théorie de la démonstration, which he defended on 11 June 1930. Its important main result is with us as Herbrand’s Theorem. Herbrand had developed strong interests in modern algebra, which was flourishing in Germany; he actually spent the academic year 1930/31 there on a Rockefeller Scholarship (and intended to spend the next year at Princeton University). In his final report to the Rockefeller Foundation he

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wrote that his stay in Germany extended from 20 October 1930 to the end of July 1931. Until the middle of May 1931 he was in Berlin, then spent a month ¨ in Hamburg and the remaining time in Gottingen. In these three cities he worked mainly with von Neumann, Artin, and Emmy Noether. Concerning his stay in Berlin he went on to say: “In Berlin I have worked in particular with Mr. von Neumann on questions in mathematical logic, and my research in that subject will be presented in a paper to be published soon in the Journal für reine und angewandte Mathematik.” The paper he alluded to is his 1931-paper, in which he compared, as his friend Claude Chevalley put it, his own results ¨ with those of Godel, i.e., the incompleteness theorems. He had learned of the first theorem from von Neumann shortly after his arrival in Berlin. In a letter of 3 December 1930, he wrote to Chevalley: The mathematicians are a very strange bunch; during the last two weeks, whenever I have seen von Neumann, we have been talking ¨ about a paper by a certain Godel, who has produced very curious functions; and all of this destroys some solidly anchored ideas. ¨ This sentence opens the letter. Having sketched Godel’s argument and reflected on the result, Herbrand concluded the logical part of his letter by: “Excuse this long beginning; but all of this has been haunting me, and by writing about it I exorcise it a little.” How did von Neumann, in November of 1930, know of a result that was to be published only in 1931? I alluded to an answer when I mentioned that ¨ Godel reported on his first incompleteness theorem at the Second Conference for Epistemology of the Exact Sciences held from 5 to 7 September 1930 in ¨ Konigsberg. On the very last day of the conference, a roundtable discussion ¨ on the foundations of mathematics took place to which Godel had been ¨ invited. Hans Hahn, Godel’s dissertation advisor, chaired the discussion and its participants included Carnap, Heyting, and von Neumann. Toward the end ¨ of the discussion, Godel made brief remarks about the first incompleteness theorem. This is the background for a more personal encounter with von ¨ ¨ Neumann in Konigsberg; Wang reported Godel’s view about this encounter in his 1981: Von Neumann was very enthusiastic about the result and had a ¨ private discussion with Godel. In this discussion, von Neumann asked whether number-theoretical undecidable propositions could also be constructed in view of the fact that the combinatorial objects can be mapped onto the integers and expressed the belief that ¨ it could be done. In reply, Godel said, “Of course undecidable propositions about integers could be so constructed, but they would contain concepts quite different from those occurring in number ¨ theory like addition and multiplication.” Shortly afterward Godel, to his own astonishment, succeeded in turning the undecidable


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proposition into a polynomial form preceded by quantifiers (over natural numbers). At the same time but independently of this ¨ result, Godel also discovered his second theorem to the effect that no consistency proof of a reasonably rich system can be formalized in the system itself.2 ¨ This makes clear that Godel did not yet have the second incompleteness ¨ theorem at the time of the Konigsberg meeting; on 23 October 1930 Hahn presented, however, an abstract containing its classical formulation to the ¨ Vienna Academy of Sciences. The full text of Godel’s 1931-paper was submitted to the editors of Monatshefte on 17 November 1930. Three days later, ¨ ¨ von Neumann wrote to Godel and characterized Godel’s first result as “the greatest logical discovery in a long time.” He went on to sketch a proof of the second incompleteness theorem, at which he had arrived independently ¨ ¨ of Godel. Godel responded almost immediately, and von Neumann assured him in his next letter that he would not publish on the subject “as you have established the theorem on the unprovability of consistency as a natural continuation and deepening of your earlier results.” However, there emerged a ¨ disagreement between Godel and von Neumann on how this theorem affects Hilbert’s finitist program. ¨ 2. Herbrand’s issue. Godel insisted in his paper that the second incompleteness theorem does not contradict Hilbert’s “formalist viewpoint:” For this viewpoint presupposes only the existence of a consistency proof in which nothing but finitary means of proof is used, and it is conceivable that there exist finitary proofs that cannot be expressed in the formalism of P (or of M and A).3 ¨ Having received the galleys of Godel’s paper, von Neumann writes in a letter of 12 January 1931: I absolutely disagree with your view on the formalizability of intuitionism. Certainly, for every formal system there is, as you proved, another formal one that is (already in arithmetic and the lower functional calculus) stronger. But that does not affect intuitionism at all. Denoting first order number theory by A, analysis by M, and set theory by Z, von Neumann continues: 2 Wang 1981, pp. 654–5. Parsons’ Introductory Note to the correspondence with Wang in ¨ ¨ Godel’s Collected Works vol. V describes in section 3.2 the interaction between Godel and Wang on which this paper is based. 3 G¨ odel 1931, p. 197; in Collected Works, vol. I, p. 195. P is the version of the system of ¨ Principia Mathematica in Godel’s 1931 paper, M is the system of set theory introduced by von Neumann, and A is classical analysis.

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Clearly, I cannot prove that every intuitionistically correct construction of arithmetic is formalizable in A or M or even in Z—for intuitionism is undefined and undefinable. But is it not a fact, that not a single construction of the kind mentioned is known that cannot be formalized in A, and that no living logician is in the position of naming such [[a construction]]? Or am I wrong, and you know an effective intuitionistic arithmetic construction whose formalization in A creates difficulties? If that, to my utmost surprise, should be the case, then the formalization should work in M or Z! ¨ Herbrand had sharpened this line of argument by the time he wrote to Godel on 7 April 1931. In the meantime he had discussed the incompleteness phenomena extensively with von Neumann, and he had read the galleys of Gödel 1931, which Bernays had given to him. On that very day, April 7, he ¨ also sent a note to Bernays and enclosed a copy of his letter to Godel. In the note he first contrasts his consistency proof with that of Ackermann, which he attributes mistakenly to Bernays: In my arithmetic the axiom of complete induction is restricted, but one may use a variety of other functions than those that are defined by simple recursion: in this direction, it seems to me, my theorem goes a little farther than yours.4 He then formulates the central issue to Bernays as follows: “I also try to ¨ show in this letter how your results can agree with these of Godle [sic].” This information puts Herbrand’s remark to Hadamard (made in early 1931) into sharper focus. Recent results (not mine) show that we can hardly go any further: it has been shown that the problem of consistency of a theory containing all of arithmetic (for example, classical analysis) is a problem whose solution is impossible. In fact, I am at the present time preparing an article in which I will explain the relationships between these results and mine. It is quite clear that Herbrand’s attempt to analyze the relationship between ¨ Godel’s theorems and ongoing proof theoretic work, including his own, ¨ prompted the specific details in his letter to Godel as well as in his paper. At issue is the extent of finitist or, synonymously for Herbrand, intuitionist methods and thus the reach of Hilbert’s consistency program. Herbrand’s let¨ ter has to be understood (and Godel in his response quite clearly did) as giving ¨ a sustained argument against Godel’s assertion that the second incompleteness theorem does not contradict Hilbert’s “formalist viewpoint.” Herbrand 4 Bernays, in his letter to Godel ¨ of 20 April 1931, pointed out that Herbrand had misunderstood him in an earlier discussion: he, Bernays, had not talked about a result of his, but rather about Ackermann’s consistency proof.


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introduces a number of systems for arithmetic, all containing the axioms for predicate logic with identity and the Dedekind-Peano axioms for zero and successor. The systems are distinguished by the strength of the induction principle and by the class F of finitist functions for which recursion equations are available. The system with induction for all formulas and recursion equations for the functions in F is denoted here by F; if induction is restricted to quantifier-free formulas, I denote the resulting system by F*. The axioms for the elements f 1 , f 2 , f 3 , . . . in F must satisfy according to Herbrand the following conditions: (1) The defining axioms for f n contain, besides f n , only functions of lesser index. (2) These axioms contain only constants and free variables. (3) We must be able to show, by means of intuitionistic proofs, that with these axioms it is possible to compute the value of the functions univocally for each specified system of values of their arguments. As examples for classes F, Herbrand considers the set E1 of addition and multiplication, as well as the set E2 of all primitive recursive functions. He asserts that many other functions are definable by his “general schema,” in particular, the non-primitive recursive Ackermann function. He also argues that one can construct by diagonalization a finitist function that is not in E, if E satisfies axioms such that “one can always determine, whether or not certain defining axioms are among these axioms.” This fact of the open-endedness of any finitist presentation of the concept “finitist function” is crucial for Herbrand’s conjecture that one cannot prove that all finitist methods are formalizable in Principia Mathematica. But he claims that, as a matter of fact, every finitist proof can be formalized in a system of the form F* with a suitable class F that depends on the given proof and, thus, also in Principia Mathematica. Conversely, he insists that every proof in the quantifier-free part of F* is finitist. He summarizes his reflections by saying in the letter and with almost identical words in 1931: It reinforces my conviction that it is impossible to prove that every intuitionistic proof is formalizable in Russell’s system, but that a counterexample will never be found. There we shall perhaps be compelled to adopt a kind of logical postulate. What is the direct consequence of the second incompleteness theorem?—The reader may recall that, under general conditions on a theory T, T proves the conditional (conT → G); conT is the statement expressing the consistency of ¨ T, and G is the Godel sentence.5 G states its own unprovability and is, by the 5 The general conditions on T include, of course, the representability conditions for the first theorem and the Hilbert-Bernays derivability conditions for the second theorem.

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first incompleteness theorem, not provable in T. Consequently, G would be provable in T, as soon as a finitist consistency proof for T could be formalized in T. That’s why the issue of the formalizability of finitist considerations plays such an important role in this discussion. 3. Gödel’s response(s). Herbrand’s conjectures and claims are much more ¨ detailed than those von Neumann communicated to Godel in his letters of ¨ November 1930 and January 1931. We know of Godel’s response to von ¨ Neumann’s dicta not through letters from Godel, but rather through the minutes of a meeting of the Schlick or Vienna Circle that took place on ¨ 15 January 1931. According to these minutes, Godel viewed as questionable the claim that the totality of all intuitionistically correct proofs is contained in one formal system. That, he emphasized, is the weak spot in von Neumann’s argumentation.6 ¨ When answering Herbrand’s letter, Godel makes more explicit his reasons for questioning the formalizability of finitist considerations in a single formal system like Principia Mathematica. He agrees with Herbrand on the indefinability of the concept “finitist proof.” However, even if one accepts Herbrand’s very schematic presentation of finitist methods and the claim that every finitist proof can be formalized in a system of the form F*, the question remains “whether the intuitionistic proofs that are required in each case to justify the unicity of the recursion axioms are all formalizable in Principia Mathematica.” He continues: Clearly, I do not claim either that it is certain that some finitist proofs are not formalizable in Principia Mathematica, even though intuitively I tend toward this assumption. In any case, a finitist proof not formalizable in Principia Mathematica would have to be quite extraordinarily complicated, and on this purely practical ground there is very little prospect of finding one; but that, in my opinion, does not alter anything about the possibility in principle. At this point, there is a stalemate between Herbrand’s “logical postulate” that ¨ no finitist proof outside of Principia Mathematica will be found, and Godel’s “possibility in principle” that one might find such a proof. By late December 1933 when he gave an invited lecture to the Mathematical ¨ Association of America in Cambridge (Massachusetts), Godel had changed his views significantly. In the text for his lecture, Gödel 1933, he sharply distinguishes intuitionist from finitist arguments, the latter constituting the most restrictive form of constructive mathematics. He insists that the known finitist arguments given by “Hilbert and his disciples” can all be carried out 6 Godel ¨ did respond to von Neumann, but his letters seem to have been lost. The minutes are found in the Carnap Archives of the University of Pittsburgh.


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in a certain system A.7 Proofs in A, he asserts, “can be easily expressed in the system of classical analysis and even in the system of classical arithmetic, and there are reasons for believing that this will hold for any proof which one will ever be able to construct.” This observation and the second incompleteness theorem imply, as sketched above, that classical arithmetic cannot be shown to be consistent by finitist means. (The system A is similar to the quantifier-free part of Herbrand’s system F*, except that the provable totality for functions ¨ in F is not mentioned. Godel’s reasons for conjecturing that A contains all finitist arguments are not made explicit.) ¨ Godel discusses then a theorem of Herbrand’s, which he considers to be the most far-reaching among interesting partial results in the pursuit of Hilbert’s consistency program. He does so, as if to answer the question “How do current consistency proofs fare?” and formulates the theorem in this lucid and elegant way: “If we take a theory which is constructive in the sense that each existence assertion made in the axioms is covered by a construction, and if we add to this theory the non-constructive notion of existence and all the logical rules concerning it, e.g., the law of excluded middle, we shall never get into any contradiction.” The proof theoretic result mentioned in Herbrand’s letter can be understood in just this way and foreshadows, of course, the central result ¨ of Herbrand’s 1931. Godel conjectures that Herbrand’s method might be generalized, but emphasizes that “for larger systems containing the whole of arithmetic or analysis the situation is hopeless if you insist upon giving your proof for freedom from contradiction by means of the system A.” As the ¨ system A is essentially the quantifier-free part of F*, it is clear that Godel now takes Herbrand’s position concerning the impact of the second theorem on Hilbert’s Program. Nowhere in the correspondence does the issue of general computability arise. Herbrand’s discussion, in particular, is solely trying to explore the limits of ¨ consistency proofs that are imposed by the second theorem. Godel’s response focuses also on that very topic. It seems that he subsequently developed a more critical perspective on the very character and generality of his theorems.8 This perspective allowed him to see a crucial open question and to consider Herbrand’s notion of a finitist function as a first step towards an answer. 7 The restrictive characteristics of the system A are formulated on pp. 23 and 24 of 1933: and include the requirement that notions have to be decidable and functions must be calculable. ¨ Godel claims, that “such notions and functions can always be defined by complete induction.” Definition by complete induction is to be understood as definition by recursion, which is by ¨ no means restricted to primitive recursion. That is made explicit in section 9 of Godel’s 1934, where “an example of a definition by induction with respect to two variables simultaneously” is discussed; an example that defines a function “that is not in general recursive in the limited sense of §2,” i.e., not primitive recursive. 8 How much the interaction with Church in 1933/34 contributed to this perspective, we can only speculate; see my paper Sieg 1997.

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4. Future Impact: Computability. The crucial open question that remained ¨ in Godel’s mind was this: For which formal theories do the incompleteness theorems hold? Just for the systems PM, ZF, and “related systems”? What is the extension of “related system”? For a fully satisfactory answer one ¨ needs a general and rigorous definition of “formal theories.” Godel points to their central features in §1 of his Princeton Lectures by saying that the rules of inference and the notions of formula and axiom have to be given constructively, i.e., for each rule of inference there shall be a finite procedure for determining whether a given formula B is an immediate consequence (by that rule) of given formulas A1 , . . . , An , and there shall be a finite procedure for determining whether a given formula A is a meaningful formula or an axiom.9 To a similar discussion of formal theories in the Cambridge Lecture he added the remark that the rules of inference are purely formal, i.e., “refer only to the outward structure of the formulas, not to their meaning, so that they could be applied by someone who knew nothing about mathematics, or by a machine.”10 ¨ Godel strove in his Princeton Lectures to make his results less dependent on particular formalisms. That is indicated even by their title On undecidable propositions of formal mathematical systems. He used, as he had done in his 1931, primitive recursive functions and relations to present syntax, viewing the primitive recursive definability of formulas and proofs as a “precise condition, which in practice suffices as a substitute for the unprecise requirement of §1 that the class of axioms and the relation of immediate consequence be constructive.” A notion that would suffice in principle was needed, however, ¨ and Godel attempted to arrive at a more general notion. ¨ In his subsequent reflections, Godel focused on the “computability” of number theoretic functions. He considers the fact that the value of a primitive recursive function can be computed by a finite procedure for each set of arguments as an “important property” and adds in note 3: The converse seems to be true if, besides recursions according to the scheme (2) [i.e., primitive recursion as given above], recursions of other forms (e.g., with respect to two variables simultaneously) are admitted. This cannot be proved, since the notion of finite computation is not defined, but it can serve as a heuristic principle.11 9 G¨ odel

1934, p. 346. 1933, p. 45. 11 Godel ¨ emphatically rejected in the sixties (in a letter to Martin Davis) that this formulation anticipates a form of Church’s Thesis: he was not convinced that his notion of recursion was the most general one. 10 G¨ odel


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What other recursions might be admitted is discussed in the last section of the ¨ Notes under the heading “general recursive functions.” Godel describes in it the proposal for the definition of a general notion of recursive function that (he thought) had been suggested to him by Herbrand: If φ denotes an unknown function, and 1 , . . . , k are known functions, and if the ’s and φ are substituted in one another in the most general fashions and certain pairs of resulting expressions are equated, then, if the resulting set of functional equations has one and only one solution for φ, φ is a recursive function. ¨ Godel went on to make two restrictions on this definition. He required, first of all, that the left-hand sides of the equations be in a standard form with φ as the outermost symbol and, secondly, that “for each set of natural numbers k1 , . . . , kn there shall be exactly one and only one m such that φ(k1 , . . . , kn ) = m is a derived equation.” The rules that were allowed in derivations are simple substitution and replacement rules. ¨ We should distinguish with Godel two novel features in this definition: first, the precise specification of mechanical rules for deriving equations, i.e., for carrying out numerical computations; second, the formulation of the regularity condition requiring computable functions to be total, but without insisting on a (finitist) proof. In his letter to van Heijenoort of 14 August ¨ 1964, Godel asserts, “it was exactly by specifying the rules of computation that a mathematically workable and fruitful concept was obtained.” When ¨ making this claim Godel took for granted that Herbrand’s suggestion had been “formulated exactly as on page 26 of my lecture notes, i.e., without ¨ reference to computability.” As was noticed earlier, Godel had to rely on his recollection, which, he said, “is very distinct and was still very fresh in 1934.” ¨ On the evidence of Herbrand’s letter, it is clear that Godel misremembered. ¨ This is not to suggest that Godel was wrong in viewing the specification of computation rules as extremely important, but rather to point to the absolutely crucial step he had taken, namely, to disassociate general recursive functions from the epistemologically restricted notion of intuitionist proof in Herbrand’s sense. ¨ Later on, Godel dropped the regularity condition altogether and emphasized, “that the precise notion of mechanical procedures is brought out clearly by Turing machines producing partial rather than general recursive functions.” At this earlier historical juncture the introduction of the equational calculus with particular computation rules was important for the mathematical development of recursion theory as well as for the underlying conceptual ¨ motivation. It brought out clearly, what Herbrand—according to Godel in his letter to van Heijenoort—had failed to see, namely “that the computation ¨ (for all computable functions) proceeds by exactly the same rules.” Godel was right, for stronger reasons than he put forward, when he cautioned in the

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same letter that Herbrand had foreshadowed, but not introduced, the notion of a general recursive function.12 Concluding remarks. What impact the introduction of the notion of general recursive function had on the development of computability theory is an equally fascinating story, which leads to a very satisfying conceptual analysis; the issue of “what precisely is finitism” is by contrast still open. The former ¨ issue was not obtained along Godelian lines by generalizing recursions, but by a quite different approach due to Alan Turing and, to some extent, Emil Post. They focused on symbolic processes underlying numerical computations instead of those computations themselves. This led to the foundations of theoretical and, via Turing’s universal machine, also of practical computer science. Consequently, those foundations emerged from what were, at the time, quite obscure quasi-philosophical issues. The general moral is, of course, that broad foundational questions can inspire concrete mathematical work, and that concrete mathematical work can call for philosophical analysis. There can be an extremely fruitful, but also subtle and delicate interplay between wide-open conceptual reflections and hard-nosed technical investigations. All of this is necessary for arriving at balanced positions. The historical evolution of the particular issues at hand confirms this and helps us to grasp their complexity. We see, finally, three specific and important points drawn from that evolution, listed in order ¨ of their increasing significance: (i) the Godel-Herbrand notion of general ¨ recursive function is really Godel’s; (ii) in the early 1930s finitist mathematics was viewed as going significantly beyond primitive recursive arithmetic; (iii) at that time, finitist mathematics was viewed as coextensive with intuitionist mathematics. Each point is counter to broadly held contemporary views and, indeed, undermines deeply held convictions concerning our logical past.13 REFERENCES

¨ [1929] K. Godel, Uber die Vollständigkeit des Logikkalküls, doctoral dissertation, University ¨ of Vienna, 1929. 12 Van Heijenoort analyzed Godel’s ¨ differing descriptions of Herbrand’s published proposals and the suggestion that had been made to him in the “private communication.” References to this work and a discussion in light of the actual letter are found in my paper Sieg 1994; see in particular section 2.2 and the Appendix. 13 This essay is based on the Introductory Notes, which I wrote for the correspondence between ¨ ¨ Godel and Herbrand, respectively von Neumann, and which are published in volume V of Godel’s Collected Works. Versions were presented at Haverford College (in October 2002), at the Institute for Advanced Study at the University of Bologna (in November 2003), and at the Special Session ¨ on Godel at the Annual Meeting of the Association for Symbolic Logic at Carnegie Mellon University (in May 2004). I am grateful to Jeremy Avigad, John Dawson, Solomon Feferman, Rossella Lupacchini and William W. Tait for perceptive suggestions, which helped to improve the paper.


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[1930] , Einige metamathematische Resultate u¨ ber Entscheidungsdefinitheit und Widerspruchsfreiheit, Anzeiger der Akademie der Wissenschaften in Wien, vol. 67 (1930), pp. 214–5. ¨ [1931] , Uber formal unentscheidbare Sätze der Principia Mathematica und verwandter Systeme I, Monatshefte für Mathematik und Physik, vol. 38 (1931), pp. 173–98. [1933] , The present situation in the foundations of mathematics, Collected Works, vol. III, (Oxford University Press, 1995), 1933, pp. 45–53. [1934] , On undecidable propositions of formal mathematical systems, Collected Works, vol. I, (Oxford University Press, 1986), 1934, (mimeographed lecture notes, taken by Stephen C. Kleene and J. Barkley Rosser), pp. 346–69. [1930] J. Herbrand, Recherches sur la théorie de la démonstration, doctoral dissertation, University of Paris, 1930. [1931] , Sur la non-contradiction de l’arithmétique, Journal für die reine und angewandte Mathematik, vol. 166 (1931), pp. 1–8. [1929] D. Hilbert, Probleme der Grundlegung der Mathematik, Atti del Congresso internazionale dei matematici, Bologna 3–10 settembre 1928, vol. 1, 1929, pp. 135– 41. [1994] W. Sieg, Mechanical procedures and mathematical experience, Mathematics and mind (A. George, editor), Oxford University Press, 1994, pp. 71–117. [1997] , Step by recursive step: Church’s analysis of effective calculability, The Bulletin of Symbolic Logic, vol. 3 (1997), pp. 154–80. [1981] H. Wang, Some facts about Kurt Gödel, The Journal of Symbolic Logic, vol. 46 (1981), pp. 653–9. DEPARTMENT OF PHILOSOPHY CARNEGIE MELLON UNIVERSITY PITTSBURGH, PA 15213, USA


¨ GODEL’S REFORMULATION OF GENTZEN’S FIRST CONSISTENCY PROOF FOR ARITHMETIC: THE NO-COUNTEREXAMPLE INTERPRETATION

W. W. TAIT

Abstract. The last section of “Lecture at Zilsel’s” [8, §4] contains an interesting but quite condensed discussion of Gentzen’s first version of his consistency proof for PA [7], reformulating it as what has come to be called the no-counterexample interpretation. I will describe Gentzen’s result ¨ (in game-theoretic terms), fill in the details (with some corrections) of Godel’s reformulation, and discuss the relation between the two proofs.

1. Let me begin with a description of Gentzen’s consistency proof. As had already been noted in [5], we may express it in terms of a game.1 To simplify things, we can assume that the logical constants of the classical system of number theory, PA, are ∧, ∨, ∀ and ∃ and that negations are applied only to atomic formulas. ¬φ in general is represented by the complement φ of φ, obtained by interchanging ∧ with ∨, ∀ with ∃, and atomic sentences with their negations. The components of the sentences φ ∨ and φ ∧ are φ and . The ¯ for components of the sentences ∃xφ(x) and ∀xφ(x) are the sentences φ(n) ¯ A ∧- or ∀-sentence, called a -sentence, is thus expressed each numeral n. by the conjunction of its components and a ∨- or ∃-sentence, called a -sentence, is expressed by the disjunction of them; and so it follows that every sentence can be represented as an infinitary propositional formula built up from prime sentences—atomic or negated atomic sentences. Disjunctive and conjunctive sentences with the components φn (where the range of n is 1, 2 or ) will be denoted respectively by φn φn n

n

Reprinted from The Bulletin of Symbolic Logic, vol. 11, no. 2, 2005, pp. 225–238. I am grateful to Jeremy Avigad, John W. Dawson Jr., and Solomon Feferman for valuable comments on earlier drafts of this paper. 1 This paper is an expansion of §4 of [18]. I regret that I did not know Thierry Coquand’s paper when I wrote [18] and so failed to cite it. Kurt Gödel: Essays for his Centennial Edited by Solomon Feferman, Charles Parsons, and Stephen G. Simpson Lecture Notes in Logic, 33 c 2010, Association for Symbolic Logic

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2. There is of course onewell-known game T (φ) associated with sentences φ of PA, played by and . Each stage of the game consists of exactly one sentence on the board, with φ on the board at the first stage. Let be on the board ata given stage. If is a prime sentence, then the game is over and is won by if the sentence is true and by otherwise. If is a -sentence, then replaces it on the board by one of its components at the next stage; and if it is a -sentence, then replaces it on the board by one of its components at the next stage. The play is over in ≤ n + 1 steps, if n is the logical complexity of φ. The truth of φ is obviously equivalent to the existence of a winning strategy for in T (φ). But, as is also well-known, there may be no effective such winning strategy. The most elementary example of this is ∀x∃y∀z φ(x, y) ∨ φ(x, z) where φ(x, y) is decidable and ∃yφ(x, y) is not. 3. Gentzen’s game G(Γ), unlike T (φ), is played with nonempty finite sets Γ of sentences, rather than with single sentences, and the plays are infinite. The game starts with Γ on the board. Let Δ be on the board at a given stage n. If Δ consists only of prime sentences, then the set on the board at stage n some composite sentences. Then + 1 is again Δ. Assume that Δ contains designates one of them, say φ. If φ is a sentence, then at stage n + 1 moves by adding of φ and possibly dropping φ from the to Δ a component set. If φ is a -sentence, then moves by adding to Δ a component of φ and possibly dropping φ from the set. wins the play in G(Γ) iff at some stage, the set on the boardcontains a true prime sentence; otherwise, wins. A winning strategy for in G(Γ) was called by Gentzen a reduction of Γ.2 It is not hard to see that the disjunction of the sentences in Γ is true if and only if there is an effective reduction of Γ. The essential difference between G({φ}) and T (φ), accounting for the effectiveness of the winning strategy in the former case, is that is allowed to keep the designated sentence n n on the board as well as some component of it, so that if he makes a bad choice of a component the first time, he has a chance later on to choose a different one. In fact, a winning strategy (not a very efficient one!) consists in going back infinitely often to designate each n n that appears on the board, so that eventually all of the n are chosen. Gentzen’s first version of his consistency proof consisted in effectively constructing, from each formal deduction of a set Γ of sentences of PA, a reduction

2 Gentzen considered, not sets of sentences, but sequents, of the form φ , . . . , φ ⇒ (n ≥ 0). n 1 We have simplified his treatment by coding this sequent by the set {φ1 , . . . , φn , }.

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of Γ.3 Such a reduction determines a well-founded tree, whose paths are precisely the plays (up to the stage at which there is a true prime sentence on the board) in accordance with the winning strategy for in G(Γ). In the published version of his first proof, Gentzen assigned an ordinal < 0 to each formal deduction, which turns out to be a bound on the height of the associated tree. The construction of the reduction proceeds by recursion on the ordinal. [14], at the cost of slightly obscuring (if you don’t read it thoughtfully) the constructive content of Gentzen’s result, recasts the construction as a cut-elimination theorem for PA with the -rule, in which the cut-free deduction obtained from the given deduction is the tree determined by Gentzen’s winning strategy.4 An original version of Gentzen’s proof, which he did not publish but is now published in [3], did not assign ordinals to deductions: in place of recursion on ordinals, it uses recursion on well-founded trees (essentially the principle of bar recursion).5 ¨ 4. Godel describes Gentzen’s proof in [8] in a way that avoids a calculus of sequents (which, as I mentioned, we have replaced by finite sets). He considers deductions of single formulas in an ordinary Hilbert-style formalization of PA. Assume that with each formula φ in the deduction we associate a certain one of its prenex normal forms ∃x1 ∀y1 . . . ∃xn ∀yn A(xi , yj ) with A(xi , yj ) = A(x1 , y1 , . . . , xn , yn ) quantifier-free and so a decidable formula. (For example, in ¬φ ∨ , bring out all the quantifiers in φ first.) Then we may as well—and, in what follows, will—regard sentences of the form A(k¯ i , m¯ j ) as prime in playing the game G({φ}). Note that a winning strategy for in T (φ) (again, taking quantifier-free sentences as prime) is uniquely determined by n functions fk (x1 , . . . , xk ) = yk of the preceding existentially quantified variables such that ∀x1 . . . xn ¬A x1 , f1 (x1 ), . . . , xn , fn (x1 , . . . , xn ) so that the system f = f1 , . . . , fn could be regarded as a counterexample ¨ to φ. Godel considers systems f , but it will be simpler for us to consider 3 In fact, Gentzen considered a ‘semi-formal’ system which, on the face of it, is stronger than PA: he admitted as axioms finitistically verifiable equations. This seems to be the system Hilbert describes in [9]. There would be no difficulty in extending the present treatment to this ‘system.’ But if we assume that any such equation is derivable in primitive recursive arithmetic, then the greater scope of his theorem is only apparent. 4 [4] contains a detailed discussion of the relation between Gentzen’s proof and Schutte’s ¨ cut-elimination theorem. 5 For a discussion of the criticism of the original version of Gentzen’s proof, leading to the published version, see [13, pp. 52–54].

¨ GODEL’S REFORMULATION OF GENTZEN’S FIRST CONSISTENCY PROOF

77

instead systems f = f1 , . . . , fn , where fk (x1 , y1 , . . . , xk ) = yk is a function of all the preceding quantified variables and so could be a Skolem function for ∃yj ∀xj+1 ∃yj+1 . . . ∃yn ¬A(xi , yj ) so that, if φ is false, then ∀x1 . . . xn ¬A x1 , f1 (x1 ), . . . , xn , fn (x1 , f1 (x1 ), . . . , xn ) . Clearly, systems f and f are obtainable from one another: to obtain f from f , define fk (x1 , y1 , . . . , xk ) = fk (x1 , . . . , xk ) and to obtain f from f, define by induction on k fk (x1 , . . . , xk ) = fk (x1 , f1 (x1 ), . . . , xk ). Whichever we start with, f and f determine the same strategy for

in T (φ).

5. If in the game G({φ}), we restrict ’s moves to those given by f, a winning strategy for effectively yields a system F = F1 , . . . , Fm of computable functionals such that ∀fA Fi (f), fj (F1 (f), . . . , Fj (f)) . On the branch of the strategy tree determined by f, find the true sentence A(k i , m j ) in the top node and set Fi (f) = ki . Gentzen’s proof amounts to a definition of this path as a functional of f by recursion on the height α of the tree, and so the Fi are obtainable directly from this proof as α-recursive ¨ functionals. Godel’s idea is to construct F directly from the Hilbert-style deduction of φ. ¨ But contrary to Godel’s assertion (p. 108) that such a system F of functionals is a reduction of φ, playing in accordance with the functions f (or f ) constitutes a restriction on the play for in G({φ}), since it could happen that the sets Δ ∪ {∀x(x)} and Δ ∪ {∀x(x)} are, respectively, an earlier and a later stage in a play, in each case with ∀x(x) designated. According to the strategy determined by f, the same component (n) must be chosen in both cases, whereas this is not required in the game G({φ}) in general. How ever, it is not hard to see that can always modify his winning strategy in the restricted game by never designating ∀x(x) more than once, so that the apparent restriction on never comes into play. In any case, we will briefly indicate in §13how a winning strategy in the form of a system of functionals F for , with restricted tostrategies of the form f, effectively determines a winning strategy even when is allowed to exercise all of its options.

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C. Parsons and W. Sieg observe in their introductory note [6, p. 82] that the winning strategy F for (in the restricted game) is precisely a witness for the so-called no-counterexample interpretation, NCI ∃F ∀fA Fi (f), fj (F1 (f), . . . , Fj (f)) of φ, first introduced in print in [12]. Kreisel showed that, from any deduction of φ, one could extract a witness. His proof is not based on the game-theoretic idea behind Gentzen’s consistency proof for PA, but rather is a corollary of Ackermann’s proof, using the -substitution method [1]: Assume given a deduction of φ. From φ logically follows ∃x1 . . . xn A x1 , f1 (x1 ), . . . , xn , fn (x1 , . . . , xn ) . In the -calculus, this has the form A t1 , f1 (t1 ), . . . , tn , fn (t1 , . . . , tn ) where the tk are -terms. Applying Ackermann’s result, the -terms are eliminated from the deduction, so that tk is replaced by some Fk (f ), and the resulting deduction is in primitive recursive arithmetic together with a principle of definition by transfinite recursion on the segment of ordinals up to some α < 0 (where α depends on the given deduction of φ). We shall see that ¨ the key lemma is the same in the two cases, Godel’s direct derivation of the NCI using Gentzen’s idea and Ackermann’s elimination of -terms, leading to Kreisel’s derivation of the NCI. (See §14.)6 6. It is easy to see that, if there is a witness G = G1 , . . . , Gn for the NCI of φ, then there is one F in standard form, i.e., such that for each k ≤ n, if fi (F1 (f), f1 (F1 (f)), . . . , Fi (f)) = gi (F1 (g), g1 (F1 (g)), . . . , Fi (g)) i ℵ1 . 1 In German the problem is frequently called “Problem der Wachstumsordnungen”. Perhaps there exists some standard English expression for it, too. 46 ¨ after praising In a letter to Stanisław Ulam of 10 February 1964 Godel, ¨ Cohen’s work, wrote similarly about the “Pantachie Problem”. What Godel was describing properly has to do with the “Scale Problem” of Hausdorff [1907, p. 152]. , the set of functions from to , can be partially ordered according to: f

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