A Note on Semantic Realism Carl G. Hempel Philosophy of Science, Vol. 17, No. 2. (Apr., 1950), pp. 169-173. Stable URL: http://links.jstor.org/sici?sici=0031-8248%28195004%2917%3A2%3C169%3AANOSR%3E2.0.CO%3B2-E Philosophy of Science is currently published by The University of Chicago Press.
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experience. The attempt to extend it to all possible knowledge leads into the realm of metaphysics. This state of affairs was well formulated long ago by Charles S. P i e r ~ e He . ~ wrote in his "Classification of Sciences": "Philosophy contents itself with observations such as come within the range of every man's normal experience-the special sciences depend upon special observation as it is given to the sense either instrumentally or by training, together with unusual diligence." By "philosophy" in contrast to the "special sciences" he obviously means "metaphysics." In order not to give rise to any misunderstanding: All scientific statements have consequences which are observable phenomena and can, therefore, be formulated in our everyday language. But if we want to formulate the principles themselves in this language we run into metaphysics.
Harvard University Collected Papers, Vol. I , Sect. 241.
A NOTE ON SEMANTIC REALISM CARL G . HEMPEL
Professor Feigl's admirably lucid and concise appraisal of the major considerations which are still significant in the controversy between realism and phenomenalism includes the important reminder that the problem a t hand should be viewed as concerning, not the truth or falsity of two conflicting hypotheses, but rather the comparative adequacy of two alternative proposals for the rational reconstruction of scientific knowledge. Feigl advocates the approach of semantic realism in preference to a phenomenalistic type of reconstruction on the grounds that in his critical examination, the latter has been "found wanting," whereas semantic realism-and only semantic realism-can provide a satisfactory systematic account of the meaning and the experiential foundation of scientific hypotheses. While I fully agree with Feigl's general appraisal of the issues involved, I have doubts as to the superiority of the proposed realistic interpretation, and I should like to state these here in outline. In closs adaptation of Feigl's own words, the central problem under discussion might be stated as follows: Do scientific hypotheses containing constructs assert anything over and above what the totality of conceivable tests would reveal in terms of evidence? And if so, what does that "something more" consist in? Feigl answers the first question in the affirmative and interprets the "surplus meaning" in terms of the factual reference of the constructs involved, as contradistinguished from the observables to which the constructs might be epistemically reducible. But what does the phrase ''factual reference" mean here? I t cannot be synonymous with the term "designatum" in the technical sense of modern semantics; for, as Feigl himself mentions, whether we may say that a term t in a language L has a designatum in that sense depends mainly on whether it is
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possible to translate t into the semantic metalanguage in which the referents of the expressions in L are dealt with. Thus, in discussing the semantics of French in English as our metalanguage, we might say that the designatum of the term "Dieu" is God, of "le nCant," nothingness, and of "l'absolu," the absolute. But presumably semantic realism would not hold that therefore those French terms have factual reference, or that their referents "really exist." Now Feigl does not offer a general definition of factual reference as understood in semantic realism, but he does state the following necessary and sufficient condition for it: " . . . in the language of empirical science all those terms (and only those terms) have factual reference which are linked to each other and to the evidential base by nomological relationships." This conception of the surplus meaning of scientific constructs and of hypotheses containing them seems to me to be quite in accord with scientific usage; in particular, it appears to do justice to the meaning of statements asserting the "reality" or "existence" of an entity or phenomenon of a kind not accessible to direct observation. Thus, e.g., the statements "This piece of metal is really gold," and "Air really consists of molecules in rapid motion" are treated, in scientific usage, as tantamount to the sentences "This piece of metal is gold," and "Air consists of molecules in rapid motion"; and these latter statements are considered sound to the extent to which they are borne out by pertinent evidence, i.e., linked to the evidential base either by purely logical connections or by means of subsidiary hypotheses which, in general, will have the character of laws. We might note here that since the connection of a scientific hypothesis with the pertinent evidence may be more or less direct, and more or less tenuous (which is another way of saying that the hypothesis may be more or less directly, and more or less fully confirmed by the relevant evidence available), the factual reference of a given construct has to be treated, strictly speaking, as a matter of degree if the intent of Feigl's criterion is to be observed. At any rate, however, the basic import of that criterion consists in the interpretation of the factual reference of scientific constructs in terms of the logical relationships which connect scientific hypotheses involving those constructs, with the evidential base of science. But those relationships can be characterized in a "phenomenalistic" reconstruction; therefore I think the latter cannot be "found wanting" in anything essential that semantic realism has to offer for the analysis of the "surplus meaning" of existential or other scientific hypotheses. I n particular, Feigl's assertion that hypotheses involving constructs do assert more than the relevant segment of the evidential basis can be adequately interpreted and duly justified in terms of a phenomenalistic analysis which does not refer a t all to the moot, and largely terminological, issue of the semantic referents of hypothetical constructs. Moreover, it can be shown that such surplus meaning is not limited to statements involving constructs, but is characteristic also of most of those generalizations which can be formulated purely in terms of observables. It will facilitate our discussion to consider this simplest case first. By an observation sentence, or evidence sentence, let us understand a sentenceno matter whether true or false--which is neither analytic nor contradictory,
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and which ascribes some directly observable characteristic to one or more specified objects, or is made up of one or more sentences of this kind by means of the logical connectives "not," "and," "or," "if . . . then." An observation sentence, therefore, contains neither quantification terms (such as "all," "some," "no," etc.) nor scientific constructs. "Rover is a swan," and "If Rover is a swan, then Rover has a yellow bill" might serve as illustrations. By an observational genera& ization, let us understand a generalized sentence (i.e., a sentence containing a t least one quantification term) which is neither analytic nor contradictory, and which, apart from logical terms, contains only observation terms, i.e., predicates representing observable properties or relations. I n particular, therefore, an observational generalization is to contain no names of individual objects. "Every dog has a heart," "Any cat is smaller than some dog," "There are white ravens," are examples of observational generalizations. Now among all the different kinds of observational generalizations, there is only one which could conceivably be said t o make the same assertion as the totality of the pertinent observational evidence. They are what Feigl calls type A laws, i.e., observational generalizations of purely universal form, such as "Every swan has a yellow bill," "All metals expand upon heating," etc. These might be considered as tantamount to the class of all their individual instances; in our first illustration, these would be all the sentences which can be obtained from the matrix "If x is a swan then x has a yellow bill" by substituting for "x" the name of any one object in the universe.' But for purely logical reasons no other kind of observational generalization asserts even a single evidence sentence. This is clear for purely existential generalizations; thus, the statement "There exist white ravens" does not enable us to conclude for any specifiable object that it is a white raven. The same verdict applies to mixed generalizations, i.e., statements beginning with a t least one universal and a t least one existential quantifier, such as: "For every thing x, there exists a thing y such that if x is a cat then y is a dog and x is smaller than y," which is a more explicit formulation of the statement that every cat is smaller than some dog. From this sentence again it is impossible to deduce any evidence sentence a t all. Hence, no evidence sentences are asserted by observational generalizations of purely existential or of mixed type. We shall therefore either have to say that generalizations of these forms have no meaning, or else we shall have to admit that the meaning of those sentences cannot be expressed by means of evidence sentences. Since sentences of the types here mentioned are used and understood in scientific and indeed in prescientific discourse, and since we wish to provide a rational reconstruction of their usage, we must reject the former alternative.-In the case of hypotheses containing constructs, the same The conception of a type A law a s equivalent with the classes of all its instances has to be taken with a grain of salt, however; for none of the language systems available forscientific use provides a name for every object in the universe, so that is is impossible actually to formulate all the evidence sentences asserted by any one given type A law.-On this point, cf. also P a r t I11 of: W. Sellars, "Realism and the new way of words," Philos. and Phenom. Res. 8 (1948) (Also reprinted in H. Feigl and W. Sellars, Readings in Analytic Philosophy, New York, 1949.)
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result applies, "only more SO,',inasmuch as the constructs are not explicitly defined by observables. MTeconclude, then, that most scientific hypotheses do not even by implication assert any observation sentences a t all, and that therefore their meaning cannot be adequately stated in terms of observation sentences alone. However, evidence sentences may be relevant for the test of a given hypothesis even though they are not entailed by it. Thus, an observation sentence stating of a certain object that it is a white raven can serve to confirm the purely existential hypothesis "There exists a t least one white raven," which is entailed by the observation sentence but does not entail it (and thus asserts less than the latter). I n an analogous, but more complex manner12a set of observation sentences may confirm a mixed generalization although it neither entails nor is entailed by that generalization. I n many cases, such a complex confirmatory relationship is based on subsidiary hypotheses which establish a connection between the hypothesis under test and the evidential basis. Hence, any attempt t o characterize the import of a scientific hypothesis exclusively in terms of potential observational evidence is bound t o prove inadequate. A more comprehensive interpretation of the meaning of theoretical statements is called for. And indeed, in order to understand a hypothesis, to know its import, we have not merely to know what evidence sentences (if any) it entails alone or in conjunction with subsidiary hyp~t~heses: we also need t o know what other, non-observational, sentences are implied by the hypothesis; what sentences-observational or otherwise-would confirm or disconfirm it, and for what other hypotheses the given one would be confirmatory or disconfirmatory. I n other words, the meaning of a statement in the language of science is reflected in the totality of its logical relationships (those of entailment as well as those of confirn~ation)t o all other sentences of that language. The logical relationships of the statement t o the evidence sentences form only part of that complex relational network which represents its meaning as here construed; and in this clear sense, the meaning of a theoretical hypothesis goes beyond what can be expressed by reference to the potentially relevant evidence alone. This interpretation of the "surplus meaning" of a hypothesis seems to me t o be in agreement with scientific usage; it calls attention to the systematic interconnections between the hypothesis on the one hand and other theoretical statements as well as observation sentences on the other; and it is precisely this logical coherence, in combination with the empirical confirmation, of a given theoretical system which to the scientist constitutes grounds for affirming the "actual existence," or "reality," of whatever hypothetical entities the system assumes. By embodying this idea, the interpretation here suggested for the "surplus meaning" of theoretical statements comes very close t o the intent of Feigl's necessary and sufficient condition for factual reference in a theoretical construct; and I think, in fact, that Feigl would agree with most of what has been said here. The only major disagreement seems to me to arise from his reliance on the 2For a detailed analysis, see R. Carnap "Testability and meaning," Philos. of Science 3 and 4 (1936 and 1937), $824-26, and C. G. Hempel, "Studies in the logic of confirmation," Mind, n.s. 54 (1945).
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notion of the reference of theoretical constructs: As I have tried to argue, this notion is unnecessary here and can be eliminated, in effect, by means of Feigl's own criterion of factual reference. I cannot, therefore, accept his contention that certain aspects of empirical science can be adequately interpreted only within the framework of semantic realism. I n further elaboration of this point, I should like to comment briefly on two specific arguments which Feigl presents apropos of the "phenomenalistic aspect of syntactical positivism." One of Feigl's strictures concerns the view that theoretical statements are equivalently translatable into compounds or classes of evidence sentences. For reasons outlined above, I agree with Feigl in considering this view as untenable. But I think that one of the basic criticisms he offers is not conclusive, and I wish t o discuss it briefly because it has the appearance of exhibiting a fundamental limitation in the methods of logical reduction. Feigl argues that since the place and/or date of the events which furnish the evidence for or against a particular existential hypothesis of type B may differ from the place and/or date of the event whose occurrence is asserted by that hypothesis, "the strict identity of reference needed for logical equivalence as a basis for genuine translation does not (generally) obtain." This criticism appears to me to imply a too narrow view of logical equivalence. Thus, e.g., the statements "June 1, 1949, falls on a Wednesday," and "June 2, 1949, falls on a Thursday" are logically equivalent (relative to some obvious definitions), and either one of them may serve as evidence for the other, although the dates mentioned in them are different. At another point, Feigl asserts that only the realistic frame permits an adequate interpretation of what we mean by the probability of existential hypotheses. Rut whatever the semantic referents of hypothetical constructs may be, surely they are not capable of independent observation; hence I cannot see how semantic realism should be able to avoid the pitfalls of probabilistic realism which Feigl has stated so succinctly-unless it should be possible to extend t o existential hypotheses the conception of probability as a logical relation between a hypothesis and a sentence reporting some evidence; but this logical concept of probability3 does not depend on the assumptions of semantic realism. The reservations which have been expressed here concerning the position of semantic realism do not, of course, imply the assertion that semantical considerations are irrelevant for the logical and methodological analysis of science. Clearly, without a semantical interpretation of a t least some of its terms, a proposed theory has no empirical import a t all, and a purely syntactical account of scientific knowledge is therefore impossible. On this point I am in full agreement with Feigl. But I do not feel convinced that reliance on the problematic concept of the factual referents of theoretical constructs is necessary or even helpful in an attempt to achieve a comprehensive and coherent theoretical account of scientific method and scientific knowledge.
Yale University aA general characterization of this concept may be found in R. Carnap, "The two concepts of probability," Philos. and Phenom. Res. 5 (1945). (Also reprinted i n H. Feigl and W. Sellars, Readings i n Analytic Philosophy, Kew York, 1949.)
