REVIEW ARTICLE THE FUNCTION DEBATE IN PHILOSOPHY Arno Wouters Institute of Biology, Leiden University, P.O. Box 9516, NL-2300 RA Leiden, The Netherlands. E-mail:
[email protected] Received 17 December 2004; accepted in revised from 8 February 2005
ABSTRACT This paper reviews the debate on the notion of biological function and on functional explanation as this takes place in philosophy. It describes the different perspectives, issues, intuitions, theories and arguments that have emerged. The author shows that the debate has been too heavily influenced by the concerns of a naturalistic philosophy of mind and argues that in order to improve our understanding of biology the attention should be shifted from the study of intuitions to the study of the actual practice of biological inquiry.
Key Words: function, functional explanation, teleology
1. INTRODUCTION In the second half of the seventies of the previous century the philosophical debate on function and functional explanation seemed to have come to an end. However, the last 15 years have witnessed to a lively interest in the topic.1 This revival did not grow out of interest in biology, but out of an interest in the philosophy of mind. The hope is that an articulation of a biological notion of function will contribute to an account of mental content. Some participants (most notably Millikan) see the notion of function they articulate primarily as a stipulated definition to be judged by its utility in solving philosophical problems. Many others claim that their definition correctly describes the use of the term in biology. I shall review this debate from the perspective of a philosopher of biology who is primarily interested in understanding the practice of biological inquiry. Such an enterprise has a value of its own and can also be useful to biologists in the same way the scientific study of the grammar of a certain language can be useful to the speakers of that language. My main conclusion will be that in order to improve our understanding of the use of function attributions in biology, the attention should be shifted from the study of intuitions to the study of the actual practice of biological inquiry. The dominant theory in the field addresses the problems of a naturalistic philosophy of mind and language but provides little insight in real biology.
1 Three important collections that represent this revival are Allen et al. (1997), Buller (1999) and Ariew et al. (2002).
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The philosophical debate on function comprises a wide range of issues and a large number of theories has been advanced to address these issues. In the remainder of this section I introduce the various perspectives on the debate, the main issues and the main theories (in that order). The different issues and theories are discussed in more detail in Section 2 and Section 3, respectively. In Section 4 I draw some conclusions concerning the desired direction of future research. The discussion of function and functional explanation plays a role in many areas of philosophy, including: • philosophy of biology (the meaning, structure, legitimacy and role of function attributions and functional explanation in biology, the implications for the status of biology as a natural science), • philosophy of the social sciences (functionalism in sociology and anthropology, functional explanation in Marxism), • general philosophy of science (the justification of functional explanation in the special sciences and the implications for our view of scientific explanation), • philosophy of medicine (the appeal to function in definitions of health and disease), • philosophy of technology (the meaning of ‘function’ in connection with technical artifacts, the role of function attributions in technology, the appeal to function in the definition of artifact), and • naturalistic philosophy of mind and language (the explanation of mental and linguistic content in terms of biological function). Given this diversity of points of view it will be no surprise that the debate is often fragmented and that different participants have different thoughts about what is really going on in the debate. Most participants would agree that a central issue is the question what justifies the use of ‘function’ in connection with some objects (such as organisms, social institutions and human artifacts) but not with others (such as physical systems). This issue is discussed in Section 2.1. However, different participants have different views on what the specific issues concerning the legitimacy of function attributions are. The dominant view (which is, as I said, shaped by the concerns of the philosophy of mind and language) starts from the intuition that functions have a peculiar feature: a function is not necessarily something an item does, but rather something that it should do. For example, my pancreas is said to have the function to produce insulin despite the fact that it actually does not do so. Function attributions are, in other words, not descriptive (they do not tell us what is the case) but normative (they tell us what should be the case). From this point of view, the main task of a theory of function is to explain how this norm arises in biological contexts. The alleged normativity of function attributions is discussed in Section 2.2. The dominant answer to the problem of normativity states that the function of something is the reason why that something is there or why that something is performed. In other words, the norm (what an item or behavior should do) is to produce the effect for which that item/behavior is there. This view is called ‘teleological’ because it appeals to what an item or behavior is for. I discuss the relation between function and teleology in Section 2.3. Biologists tend to be wary of teleological notions. In connection with artifacts ‘what it is for’ can be spelled out as the reason why it was put there by a designer. This is not acceptable in connection with biological function. However, naturalistic philosophers point out that in biology ‘what it is for’ can be spelled out as ‘what it was selected for’ (e.g. for which effect it was maintained in the process of natural selection). The
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theory that it is the function of an item or behavior to produce the effects for which it was selected in the past and which explain its current presence is called ‘the selected effects theory of function’ (Section 3.5). It is the dominant theory in the field. However, if one seeks to understand biology the selected effects theory has many problems. One problem is that this theory defines ‘function’ in evolutionary terms, whereas biologists tend to distinguish between functional biology and evolutionary biology. The relation between function and evolution is discussed in Section 2.4. From the point of view of understanding biology the main issue is the question how function attributions are used in the quest for knowledge in biology. This issue is introduced in Section 2.5. It has received far less attention in the debate than the other ones. Roughly speaking, one may distinguish five main approaches to an analysis of function: the systemic approach (Cummins, 1975, 1983; Prior, 1985; Craver, 2001; Davies, 2001), which defines function as a contribution to a capacity of a complex system (Section 3.2); the goal contribution approach (Nagel, 1961; Boorse, 1976, 2002; Adams, 1979; En¸c and Adams, 1992), which defines function as a contribution to the achievement and/or maintenance of a goal state (Section 3.3); the life chances approach (Canfield, 1964, 1965; Ruse, 1971; Wimsatt, 1972; Bigelow and Pargetter, 1987, Horan, 1989), which sees functions as effects that enhance the life chances of their bearers (Section 3.4); the etiological (or historical) approach (Neander, 1980, 1983, 1991a,b; Millikan, 1984, 1989b; Mitchell, 1989; Brandon, 1990; Griffiths, 1993; Godfrey-Smith, 1994; Buller, 1998), which sees functions as past effects that explain the current presence of the function bearer (Section 3.5); and the non-historical selection approach (Kitcher, 1993; Walsh, 1996), which sees functions as effects for which the function bearer is selected (in certain circumstances) (Section 3.6). As an example consider the function of the heart to propel the blood (which is the philosopher’s favorite example and quite often the only one mentioned). According to the systemic approach, propelling the blood is a function of the heart because that is the heart’s role in the complex machinery that maintains the organism’s state of being alive. According to the goal contribution theory propelling the blood is a function of the heart because that is how the heart contributes to the organism’s goal to survive and reproduce. According to the life chances approach propelling the blood is a function of the heart because that is what the heart does that results in the fitness of its bearers being higher than that of hypothetical organisms in which these activities would be lacking or replaced. According to the etiological approach propelling the blood is a function of the heart because that is how hearts in the past contributed to their maintenance in the population which explains their current presence. Finally, according to the non-historical selection approach propelling the blood is a function of the heart (of the organisms of a certain population) because propelling blood is why hearts are maintained in that population. As said, the favorite kind of theory of contemporary naturalistic philosophers of mind and language is selected effects theories (especially those of Neander, 1980; 1983, 1991a,b; Millikan, 1984, 1989b; Godfrey-Smith, 1994). Selected effect theories are etiological theories of function according to which it is the function of an item or behavior to bring about the effects for which it was selected in the past. For example, according to selected effect theories it is the function of our heart to pump blood because pumping blood is what our ancestors’ hearts, in the (possibly recent) past, did that caused those ancestors to be selected over competitors in which the heart had a different form (or was lacking altogether).
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2. ISSUES 2.1 When is Function Talk Appropriate? Why? Function talk is appropriate only to some domains of the world. The parts and behaviors of living organisms are thought to have functions; the parts of technical artifacts such as watches, cars and bridges have functions; the artifacts themselves have functions; human actions and social institutions can have functions; grammatical devices, such as nouns and tenses, have functions; but most people are not prepared to attribute functions to the parts of “purely physical systems” (Nagel, 1961: 406) such as a volume of gas, or a solar system. This observation raises several interesting philosophical questions: • What is it that makes it appropriate to talk of functions? How do systems of which the parts have functions differ from systems of which the parts do not have functions? • Can all talk of functions and purposes be treated along the same line? Is there a unifying phenomenon that underlies all function talk? Or is function talk in biology fundamentally different from function talk in connection with artifacts and human behavior? In connection with the first question the main lines of answer are: 1. function talk is appropriate when applied to parts and activities of organized systems (functions are roles in an organization), 2. function talk is appropriate in connection with systems that have goals or purposes (functions are means to ends), 3. function talk is appropriate with regard to systems that result from a process of reasoned design (functions are the reasons why something is there), 4. function talk is appropriate when applied to parts of systems that have a selection history (functions are the reasons why something is there). Theories of function should, of course, detail what organized systems, goals, reasons, and so on, are and how such systems can be recognized. Of course it is perfectly legitimate to hold that one answer applies to one domain of the world and another answer to other domains. It seems, for example, quite natural to combine the view that function talk in connection with organisms is appropriate because of their organized character or because of their selection history, with the view that talk of functions in connection with technical artifacts is appropriate because these things are designed for a certain goal or reason. However, quite a few philosophers aim for a unified theory that applies to all domains in which function-talk is appropriate. Wright (1973, 1976) sees a “consequence etiology” as the unifying principle: both artifacts and parts of organisms are there because of their consequences. Boorse (1976, 2002) sees goal-directedness as the common phenomenon. Millikan (1984) maintains that the phenomenon of selection underlies all function talk. Kitcher (1993) and Dennett (1995) mention design as the unifying phenomenon. Dennett goes as far as to argue that the process of evolution by natural selection is, in essence, a process of (unconscious and automatic) reasoned design.
2.2 Normativity Perhaps, the most central issue in the current debate about function is the normativity of functions. Intuitively, a function is something that an organ is supposed to do and an
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organ has that function even if it fails to do what it is supposed to do. In that case it is said to malfunction. For example, a heart is supposed to pump blood and it malfunctions if it fails to do so. Furthermore, the function of an organ seems to set a norm for what it is to function adequately and for what counts as better and worse. A heart does not perform its function well enough if it pumps the blood around but in a manner that is too slow for the organism to provide the inner organs with enough oxygen. A theory of function is supposed to explain how such biological norms arise out of a physical world (i.e. why it is legitimate to expect from a heart that it pumps blood) (see especially Millikan, 1984: 17–19, 1989b; Neander, 1991a: 180–183). It is because of this alleged normative feature of biological functions that naturalistic philosophers of mind and language are so much interested in the notion of biological function. Naturalistic philosophers aim to account for mental and linguistic phenomena, such as content or meaning, in terms of entities, processes and mechanisms accepted by the natural sciences (such an account is called a ‘naturalization’ or ‘reduction’). Two important features to be accounted for are aboutness (our thoughts and words are usually about something – I may, for example, think or talk about my socks) and misrepresentation (my thought that my socks are green is a thought about my socks, although in fact my socks are red). A theory of content should explain how representation is possible (what makes my thought or claim that my socks are green a thought or claim about my socks?), while allowing for misrepresentation (how can false thoughts or claims about my socks be thoughts or claims about my socks). Naturalistic philosophers (e.g. Millikan, 1984, 1986, 1993b; Papineau, 1987; Dretske, 1988) expect that, because of its alleged normative features, the biological notion of function will enable them to provide such an account in purely naturalistic terms (see Neander, 2004 for an excellent overview). There are two ways to account for the alleged normativity of function attributions. One is to take recourse to what is statistically normal and define function in terms of what a trait typically does. However, as Millikan (1989b: 295) and Neander (1991a: 182) have pointed out, the performance of a function is often not statistically normal (see Section 3.2). The other is to take recourse to teleology and define the function of a trait in terms of the reason why that trait is there. Teleology is discussed in the next section (Section 2.3). A third position rejects the idea that function attributions in biology are normative (Wouters, 1999; Davies, 2001; Boorse, 2002) (see Sections 3.2 and 3.3). The issue of the normativity is often associated (some will say confused) with two other issues. One is the need for categories that allow biologists to identify organs between individuals and between species. My heart is the same kind of organ as your heart and the hearts of the different species of vertebrates are all hearts. Millikan (1984: 17–19, 1989a: 295) and Neander (1991a: 180) take it for granted that cross-individual and crossspecies identification is done by means of functions. They combine this unsubstantiated assumption with the observation that some hearts are malformed and not able to pump blood, to derive the conclusion that a function is not something a thing actually does or is capable of doing, but rather something a thing is supposed or designed to do. How else could a malformed heart be a heart? Amundson and Lauder (1994) and Wouters (1999: Section 7.3.3) argue that this argument rests on a misunderstanding. Cross-individual and cross-species identification of organs is grounded in homology (evolutionary origin) rather than common function: a malformed heart is a heart not because it is supposed or designed to pump blood, but because it is derived from the same organ as other hearts in
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a common ancestor. Hence, there is no reason why a theory of function should account for cross-individual and cross-species identification. The other issue that is often associated with normativity is the difference between ‘having a function’ and ‘performing a function’. Biologists routinely say that the function of the vertebrate heart is to pump blood, although they are well aware that there regularly turn up individuals in which the heart does not do so. According to Millikan (1984: 17– 19, 1989a: 52) and Neander (1991a: 180) this is evidence that a function is not something an item actually does, but rather something it is supposed or designed to do. Malformed hearts have a function, but they do not perform it. Wouters (1999: 216–218) and Boorse (2002) maintain that one need not take recourse to a notion of malfunction in order to explain the difference between ‘having a function’ and ‘performing a function’. Wouters argues that the view of Millikan and Neander (in which ‘performing a function’ is dependent on ‘having a function’) puts the cart before the horse. According to Wouters, general function statements of the form ‘item/behavior i of s-organisms has function f’ are just vague generalizations about the functions item/behavior i performs in s-organisms. Saying that an item has a function it does not perform, is just another way of saying that that item does not perform the function homologous items of related organisms perform. The existence of exceptions to generalizations is not specific to functions. For example, biologists routinely say that the mammalian heart has four chambers, although they are well aware that there regularly turn up individuals in which the separation is incomplete. The logic, epistemology, and semantics of such statements is an interesting issue in the philosophy of science, but there is no reason to expect an answer from a theory of function. In a similar vein, Boorse (2002) argues that the distinction between weak function attributions (‘trait t serves/performs function f’) and strong ones (‘the/a function of t is f’, ‘t has function f’) is just a loose matter of how consistently and how frequently a certain function is performed. Weak functions are performed occasionally or by accident, strong functions are consistently and typically performed. Furthermore, according to Boorse, one should carefully distinguish between two kinds of strong functions: individually typical functions and species typical functions. The species typical function of human hearts can be to pump blood, although pumping blood is not the function of a certain malformed heart.
2.3 Teleology A long standing issue with regard to biological function is the relation between function talk and teleology (the idea that one can explain things by appeal to their purpose). In the fifties, sixties and seventies, teleology used to have a bad name both in biology and in philosophy. The prevailing view was that teleological explanations are illegitimate outside the context of human intentional action. In pre-Newtonian physics teleology was acceptable as a means to explain the direction of motion. For example, the direction of a falling stone was explained on the assumption that the stone moves towards its goal at the centre of the Earth. Such explanations were gradually eliminated from the physical sciences in the course of the 17th century. In the life sciences teleology remained acceptable to explain adaptation, until Darwin developed the means to explain adaptedness in a radical non-teleological way. Unfortunately many biologists failed to appreciate this achievement and teleology ran rampant, in the form of vitalism, Lamarckism and orthogenesis, until the Evolutionary Synthesis put the definitive end to teleology. Nevertheless,
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biologists did not cease to use expressions like ‘the function of’, ‘the role of’, ‘serves as’, ‘in order to’, ‘for the sake of’, and ‘for the purpose of’; expressions that are, at first sight at least, teleological in character. This posed a problem to philosophers of science with an interest in biology. How can function talk be explanatory in absence of purposes? Is such talk teleological indeed? If so, in what sense? Shouldn’t it be banned? If it is not teleological, what is its use? Is it just a relict of the past that will erode in the future? From the eighties onwards, teleology became more and more accepted among philosophers, especially after Neander (1980) and Millikan (1984) pointed out that the theory of evolution by natural selection justifies a certain kind of teleology. As selection operates on certain effects it makes sense to ask for which effects a trait was maintained in the population. Although the trait was not brought about in order to produce these effects, it has been maintained because it had those effects and those effects can, hence, be seen as the reason why that trait is there and as what the trait is for. For obvious reasons, I shall call this kind of teleology ‘maintenance teleology’. This notion of teleology enables naturalistic philosophers to account for the normative character of function attributions. If function is defined as the effects for which a trait was selected in the past, the function of a trait is what that trait is for (even if it does not do that or is not able to do that). In the course of history, roughly speaking, six positions on the dependence of function talk on teleology have emerged: 1. Intentional teleology: the parts and processes of living organisms owe their functions to the intentions of their divine creator. This was the position of 19th century British natural theology (Paley, Whewell and others), the creationist position against which Darwin opposed. 2. Immanent teleology: things that have functions are brought about in order to perform their function, but (in contrast with the previous view) it is not assumed that the production of functional and adapted beings is consciously or intentionally organized. The goal is sought because of its attractiveness and the organization needed to reach the goal is brought about internally (by the developing organism itself) in order to satisfy its own needs, rather than forced by an external organizer in order to reach his goals. The idea of immanent teleology originates in the works of Aristotle (384–322 B.C.). In Darwin’s time it was the position of the German zoologist Karl Ernst von Baer (1792–1876) and his followers who sought the explanation for life’s adaptedness in the teleological character of the process of ontological development (see Lenoir, 1982). 3. Maintenance teleology: function talk derives its meaning from the selective character of the process by means of which the things that have functions develop. Although things that have functions are not necessarily brought about in order to fulfill the function, such things are maintained (selected) for what they are doing and their past selection explains their current presence. In other words ‘what it is for’ should be read as ‘what it was selected for’ and this explains ‘why it is there’ (Millikan, 1984; Neander, 1991b; Brandon, 1990; Kitcher, 1993). Lennox (1993) has argued that this was Darwin’s own view of function talk and adaptation. 4. Organizational teleology: function talk derives its content from the way in which organisms are organized. Function talk is useful to explain the manner of organization, but it is not implied that the organization or its elements were brought about in order to perform that function. We can say of an organized system, its parts and activities what they are for, but this ‘what it is for’ did not necessarily
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play a role in bringing about the organization and does not causally explain why it is there (Nagel, 1961; Boorse, 1976, 2002; Wouters, 1999; Craver, 2001). 5. As if teleology: function talk in biology is merely metaphorical (Woodfield, 1976; Nissen, 1993; Schaffner, 1993: chapter 8, Ruse, 2002). This idea is usually traced back to the famous German philosopher Immanuel Kant (1790). 6. Merely linguistic teleology: the teleological character of function talk is merely apparent. It can be translated without loss of asserted content into non-teleological talk of necessary and/or sufficient conditions (Hempel, 1959; Nagel, 1961, 1977).
2.4 Function and Evolution An interesting issue from the point of view of understanding biological science is the relation between function attributions and evolutionary explanations. The dominant theory of function in naturalistic philosophy, the selected effects theory, defines the notion of function in terms of past selection and sees function attributions as a kind of historical selection explanations. This close association of function with evolution gives rise to several problems. A first problem concerns the pre-Darwinian origin of the notion of function. The notion of function can be traced back to Aristotle (384–322 B.C.) and is, thus, much, much older than the idea of evolution by natural selection (which stems from the middle of the 19th century). Pre-Darwinian functional biologists such as Harvey (1578–1657) and Cuvier (1769–1832) applied the same principles in functional reasoning as do modern functional biologists. Neander (1991a) claims, in defense of her selected effects theory, that the meaning of the term ‘function’ changed after the advance of Darwin’s theory of evolution, but she does not provide evidence for this claim. A second problem has to do with the distinction between function attributions and evolutionary explanations. Selected effect theories see function attributions as a kind of evolutionary explanation. This would make the study of function a branch of evolutionary biology. However, biologists routinely distinguish between functional explanations and evolutionary explanation and important discussions of biological explanation by biologists tend to make an explicit distinction between the study of function and the study of evolution. For example, the systematic biologist Ernst Mayr maintains that biology consists of two largely separated fields: functional biology (the study of individual level mechanisms) and evolutionary biology (the study of the history of those mechanisms) (Mayr, 1961, 1993, 1997). Others (e.g. Futuyma, 1997) speak of functional biology and evolutionary biology as two “modes” or “ways” of doing biology. Niko Tinbergen, one of the founding fathers of ethology, distinguished four issues in biology: causes and mechanisms, function, development, and evolution (Tinbergen, 1963). A third problem has to do with the practice of attributing functions. If the concept of function were historical in nature, one would expect that biologists base their verdict concerning the function of an item or behavior on historical studies. In fact, functions are often attributed in absence of any historical study (for examples see Kitcher, 1993; Amundson and Lauder, 1994; Wouters, 1999). Indeed, many biologists would argue that although functional explanations can stand on their own, evolutionary explanations depend on a preceding functional explanation (e.g. Bock, 1999). One alternative to the historical view is the view that function attributions are not of a historical nature. Note that the proponents of non-historical theories of function do not
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necessarily deny that function attributions are important in evolutionary explanations. They simply maintain that such explanations employ a non-historical concept of function. As Amundson and Lauder (1994), Walsh (1996), Ghiselin (1997: 289), Wouters (1999: section 5.5) and Boorse (2002) point out, nothing precludes the use of a non-historical concept of function in a historical context. A third position is the view that the notion of function as it is used in functional biology differs from the one used in evolutionary in biology. The functional notion is non-historical in character, the evolutionary one is historical. The functional notion is used to explain complex activities and the evolutionary one to explain trait presence. This dualistic view was first proposed by Millikan (1989a) and has become widely accepted in philosophy (see Section 3.7). The main problem with this view is its failure to account for the many interconnections between functional and evolutionary studies. The dualistic view implies that the evolutionary function attributions are the result of historical studies. In fact, functions are attributed on the ground of functional considerations and such nonhistorical functions are subsequently used in evolutionary studies.
2.5 Functional Explanation In regard to functional reasoning (usually called ‘functional explanation’) two issues should be distinguished. The first concerns the structure of this kind of reasoning, the second concerns the question of what such kinds of reasoning add to scientific understanding. I discuss these issues in that order. 2.5.1 The structure of functional explanations The structure of functional explanations (that is of reasonings that purport to explain a trait by appeal to its utility) has not received much explicit attention in the recent debate. Roughly spoken there are two positions.2 The first is the view that functional explanations infer a conclusion to the effect that certain organisms (e.g. vertebrates) (must) have a certain trait (e.g. a beating heart) from two kinds of premises, namely statements stating that certain conditions apply to the organisms in question (e.g. that they are alive and healthy) and statements expressing laws (e.g. ‘vertebrates need a circulatory system to be alive and healthy’) (Hempel, 1959; Nagel, 1961; Pap, 1962; Ruse, 1971, 1973; Nagel, 1977; Horan, 1989). The second is the view that functional explanations merely consist of a function attribution in answer to a why-question (e.g. the statement ‘the heart of vertebrates has the function to circulate the blood’ in answer to the question ‘why do vertebrates have a heart?’). The latter view is defended by Canfield (1964) and assumed by many others. The idea that function attributions are intrinsically explanatory is at the heart of the etiological approach (see Wright, 1973, 1976; Millikan, 1989a; Neander, 1991b). Kuipers (1986, 2001: Section 4.2) and Kuipers and Wisniewski (1994) appear to agree with Canfield that a functional explanation answers a why-question merely by attributing a function to the trait to be explained. However, unlike Canfield and many others, Kuipers
2 Many proponents of the systemic account deny the existence of functional explanations in this sense.
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and Kuipers and Wisniewski agree with the intuition of Hempel (1959) and Nagel (1961, 1977) that a functional explanation is a reasoning process. However, Kuipers and Kuipers and Wisniewski do not reconstruct that reasoning as a deductive-nomological argument. According to Kuipers and Kuipers and Wisniewski the reasoning process that constitutes a functional explanation is the hypothetico-deductive reasoning that establishes the function attribution that answers the original request for explanation. Wouters (1999, forthcoming) shows by means of many examples from real biology that the structure of the answer to a request for functional explanation is much more complex than Canfield, the proponents of the etiological approach, Kuipers, and many others assume. According to Wouters, functional explanations address a question of the form ‘why is it useful that item/behavior i of s-organisms has character c1 rather than c2 ?’. The answer (1) identifies a biological role f of i, (2) explains why it is useful to perform f, and (3) explains why it is more useful to perform f by an i with character c1 than by an i with character c2 . For example, Lannoo and Lannoo (1993) address the question why electric fishes often swim (i) backwards (c1 ), rather than forwards (c2 ). Their answer is that (1) backwards swimming (i) serves to scan potential prey (f), (2) scanning (f) is useful because it compensates for the lack of focus of the electric sense, and (3) it is useful that the scan (f) is done by swimming (i) backwards (c1 ) rather than forwards (c2 ) because a forward scan would put the fish in a bad position to catch the prey after the scan. This answer relates the habit of electric fishes to swim backwards to the fact that these fishes use an active electric sense to detect prey. The function attribution ‘swimming backwards serves to scan potential prey’ is only the first part of this complex explanation. 2.5.2 The explanatory force of functional reasoning The question of what functional reasonings add to scientific understanding is usually discussed in the context of one or both of two general theories of explanation: the inferential theory (Hempel and Oppenheim, 1948) and the causal theory (Salmon, 1984). According to the inferential theory explanations yield the insight that the phenomenon to be explained was to be expected in view of the explanatory facts. This is done by deriving (deductively or inductively) a description of the phenomenon to be explained from the laws of nature together with statements describing the conditions in which this phenomenon occurs. For example, one may show that a certain solar eclipse was to be expected by inferring the conclusion that the light of the Sun cannot reach the Earth from a description of the positions of Sun, Moon and Earth, and the laws about the propagation of light. In connection with function, inferential accounts have to face the problem of functional equivalents: there are often different ways to fulfill a certain task or meet a certain requirement (for example, blood circulation, tracheae and water transport provide different means to meet the need for a system to transport oxygen). Hence, from the fact that a certain organism is able to fulfill a certain task or meet a certain requirement one may not infer the presence of a particular part or behavior (Hempel, 1959; Nagel, 1961, 1977). According to the causal theory of explanation, explanations provide insight into how the phenomenon to be explained is brought about by the explanatory facts. According to this theory, the insight provided by the explanation of the solar eclipse concerns how this phenomenon was brought about (how the eclipse results from the Moon sliding between the Sun and the Earth). In view of the causal theory of explanations, functional
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explanations are problematic because at first sight, functions are effects of the part or activity to be explained and effects do not bring about their causes.
2.6 Conclusion It will be clear from the discussion of the main issues in the philosophical analysis of function that there is a tension between the concerns of naturalistic philosophy of mind and language and those of philosophy of biology (and more generally philosophy of science). In the first, the central problem of a theory of function is the problem of normativity and this problem is solved by means of a teleological and evolutionary notion of function. In the second, the central problem is to understand how function talk contributes to science. A teleological and evolutionary notion of function is of little help in this area, because such a notion plays virtually no role in biology. Given the diversity of concerns one might wonder whether one theory would fit all concerns. I return to this issues in Section 3.7 after discussing the different theories of function.
3. THEORIES OF FUNCTION 3.1 Intuitions About Function Philosophical analyses of function typically proceed on the assumption that a function is a special kind of effect. A philosophical theory of function is supposed to provide criteria for distinguishing effects that are functions from effects that are not. The theories are judged by the extent to which they reproduce our intuitions about which effects are functions and which not. In the course of the debate a large number of intuitions have been presented about the requirements that an adequate notion of function should satisfy. Of course, not everyone shares these intuitions, but most philosophers would agree that the following intuitions cannot be ignored. A serious philosophical theory of function should either meet these requirements or else its proponents are supposed to explain why it is not important, or even a virtue, that it does not do so. 1. A theory of function should distinguish between activities that are functions (such as the beating of the heart) and activities that are side-effects of functional organs (such as heart sounds and pulses) (Hempel, 1959). 2. A theory of function should not allow one to ascribe functions to parts of systems that are not believed to have parts with functions (such as our solar system) (Nagel, 1961: 406). 3. A theory of function should allow for maladapted functions: one function of a polar bear’s fur is to reduce heat loss, both when it lives in its natural artic environment and when it resides in a zoo in the tropics (Munson, 1972); the wing muscles of birds have the function to enable flight, even in small birds on stormy islands where flight is detrimental to survival (Hinde, 1975) (see also Cummins, 1975: 755–756). According to Neander (1983: 89) changes in the environment over the entire range of a species can even lead to species wide maladaptation. Schaffner (1993: 388, 399) and Boorse (2002: 85), on the other hand, maintain that maladapted functions are counterintuitive.
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4. A theory of function should not depict the use other organisms make of the items of a certain organism as functions of those items. It is, for example, not a function of a dog’s long hair to harbor fleas (Ruse, 1973: 183). 5. A theory of function should distinguish between effects that are functions and effects that are accidentally useful. Although belt buckles occasionally save their wearers’ life by deflecting bullets, it is not a function of belt buckles to deflect bullets (Wright, 1973: 147). A fox hunting for a pheasant may accidentally hit upon a berry shrub but finding that shrub was not the function of the hunt (adapted from Millikan, 2002: 137). 6. A theory of function should not depict the systematic use humans make of existing items for new purposes as functions of those items. It is, for example, not the function of the human nose to support eyeglasses (Wright, 1973: 148), neither is it the function of heart sounds to facilitate the diagnosis of heart diseases (Neander, 1983: 80/81). Mahner and Bunge (1997: 158), Wouters (1999), and Boorse (2002), on the other hand, observe that the exploitation of an existing item for a new function is a common evolutionary scenario (this scenario is known as a change of function and was already known to Darwin, 1859). The use humans make of their nose to support glasses is no exception and should, for that reason, count as a function of human noses. 7. A theory of function should allow one to attribute functions to traits that currently do not vary in the population (Hinde, 1975). 8. A theory of function should distinguish currently functional items from vestiges (like vestigial eyes in cave dwellers) (Wright, 1976: 87, Griffiths, 1993). 9. A theory of function should allow one to attribute functions to the parts and behaviors of so-called ‘instant organisms’, hypothetical organisms that have no evolutionary history. For example, the heart of a lion that came into existence because all its components accidentally and instantaneously fell together, would still have the function to propagate the blood (Boorse, 1976: 74; En¸c, 1979: 362). 10. A theory of function should enable us to attribute functions to items that do not actually perform it (most sperm cells will never fertilize an egg cell and mating displays quite often do not have the intended effect) (Millikan, 1984, 1989a). 11. A theory of function should enable us to attribute functions to items such as malformed hearts that are incapable of performing their function (Millikan, 1984, 1989a; Neander 1991a). Neander (1991a: 182) even maintains that in the case of epidemics and major disasters malfunction can become widespread within a population. 12. A theory of function should allow one to attribute functions to the parts and behaviors of sterile organisms such as mules (Schaffner, 1993: 388). 13. A theory of function should not allow one to attribute functions to organisms as a whole (Godfrey-Smith, 1994). 14. A theory of function should not allow one to attribute functions to such things as junk DNA, selfish DNA, and segregation distorter genes (Godfrey-Smith, 1994). 15. A theory of function should allow one to attribute functions to traits that are selected against (Wouters, 2003). For example, it can be a function of the pattern of wing coloration of certain moths to provide camouflage, even if variants exists with another pattern that confers a higher relative fitness because it provides better camouflage.
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In the remainder of this section I discuss the different theories of function in the light of these intuitions.
3.2 The Systemic Approach According to the systemic approach the function of an item is the role of that item in bringing about an activity or capacity of a complex system of which that item is a part. For example, on this view it is the function of the heart to pump the blood because pumping the blood is how the heart contributes to the circulation of the blood. This sense of ‘function’ was originally described by Bock and Von Wahlert (1965), who proposed to call functions in this sense ‘biological roles’. The classic statement of this view in philosophy is that of Cummins (1975, 1983: chapter 2). According to Cummins, function attributions owe their usefulness and legitimacy to a certain strategy of explanation which he calls “explanation by functional analysis” or simply “analytic explanation”. Such explanations explain a capacity of a system by analyzing that system into a number of components, the organized activity of which results in the capacity to be explained. The power of this strategy of explanation depends on the extent to which the capacities of the parts are simpler than and different from the capacity to be explained, and on the relative complexity of the organization attributed to the system (reflected among others in the degree to which the tasks of the parts differ from each other). The systemic theory allows the attribution of functions to maladapted traits (intuition 3), as long as those traits help to maintain an important capacity.3 The systemic theory also allows the attribution of functions to traits that do not vary (intuition 7), traits of instant organisms (intuition 8), traits of sterile organisms (intuition 12) and traits that are selected against (intuition 15). Wouters (1999, Section 2.2.2) provides many examples to show that this notion of function is central to a large part of functional biology. The two main objections raised against this view are the ‘too liberal’ objection and the objection that this account does not provide the right sort of normativity. A related objection is that of ‘indeterminateness’. The ‘too liberal’ objection states that the systemic account fails to meet intuition (2) above: it would allow the attribution of functions to parts of systems of which we intuitively think that their parts don’t have functions. Cummins’ account would for example allow us to say that it is the function of mists to create rainbows (Bigelow and Pargetter, 1987: 184), of clouds to make rain (Millikan, 1989b: 294), of tectonic plate movements to produce earthquakes (Neander, 1991a: 181), of rocks in a river to widen the delta, and of dirt in a pipe to direct the flow (Griffiths, 1993: 411). Moreover, the
3 This might seem a mixed blessing. Schaffner (1993: 388, 399) and Boorse (2002: 85), for example, reject the notion of maladapted function altogether. Others might intuitively agree that the wing muscles of island birds have the systemic function to enable flight, but the fur of Munson’s polar bear in the zoo seems more problematic. Although the fur is clearly involved in temperature regulation, the poor animal would probably have less difficulty to maintain its temperature if it had less fur. It depends on the definition of ‘contribution’ whether or not preventing heat loss would count as a systemic function. If ‘contribution’ means ‘causally involved’ the systemic function is indeed to prevent heat loss and to some this might be counterintuitive.
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systemic account would, in violation of intuition (1), fail to distinguish properly between function and side effect. For example, on Cummins’ account it would be a function of a certain tumor to exercise pressure on a certain artery in the brain (this explains the organism’s capacity to die of cancer) (Neander, 1991a: 181) and of the heart to weigh what it does (this contributes to the heart’s capacity to tip the scales on a certain number of pounds) (Sober, 1993: 86). Finally, the systemic account would violate intuition (6) as it would allow one to attribute to the liver the function to house flukes (this explains the capacity to die of fluke infestation) (Griffiths, 1993: 411). Amundson and Lauder (1994) and Wouters (1999: Section 5.6.2) argue that this objection is exaggerated. None of Cummins’ critics details the analytic explanation that would justify the counterintuitive function attribution in question. Most of the examples can be rejected outright, on the ground that according to Cummins’ criteria there is no reason to apply the analytic strategy of explanation. For example, the capacities of the parts of the heart that would explain the heart’s weight are neither more simple than and different from the capacity to be explained nor different from each other. In those cases (such as the function of rain in the Earth’s water cycle) in which there is reason to assume that the analytic strategy is useful, the relevant researchers do not hesitate to speak of functions. Craver (2001) provides the definite answer to the ‘too liberal’ objection. In his view, function attributions describe how an item participates in the activity of a mechanism. Roughly speaking, mechanisms are complex systems that are organized in such way that they produce a regular activity. The term ‘organized’ refers to the actual characteristics of the system, not to the way in which it was produced. A mechanism is organized for a particular activity if the performance of that activity critically depends on the hierarchical and spatial arrangement of the parts of the mechanism and the order and timing of their activity. The heart is organized for pumping the blood because pumping the blood critically depends on its internal structure, on its relation to the vena cava, the pulmonary artery, the pulmonary vein and the aorta, on the order and time of the contraction of its chambers and so on. The heart’s capacity to tip the scales on a certain number of pounds, on the other hand, does not critically depend on the heart’s structure or the timing of its activity and for that reason the heart is not organized for that activity. The second main objection to the systemic account says that this account does not provide the sort of normativity that function claims are supposed to have. If someone’s heart is malformed and fails to pump the blood around we are inclined to say that the heart fails to perform its function. However, on the systemic account this seems impossible: if the heart of certain individual does not pump the blood, its pumping blood cannot explain the circulation of the blood (if there is circulation at all).4 Hence, in violation of intuition (11) on the systemic account pumping blood cannot be the function of a malformed heart (Millikan, 1989b: 294; Neander, 1991a). Two kinds of answers have been provided to this objection. One kind takes recourse to what is typical or normal for the type of item in question. This is the route taken by Prior (1985). Prior distinguishes a special subset of functions, the s-functions, which are functions that standardly contribute to the survival and/or reproduction of the organisms
4
This, at least, is the received view, that seems to be endorsed by Cummins (1975: 757) himself.
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that perform it. Malfunction is the failure to perform an s-function. Millikan (1989b) and Neander (1991a) criticize this kind of move, among others on the ground that performance of the function is often not normal: most mating displays fail to attract a mate and most sperm cells will never fertilize an ovum (intuition 12). The second kind of response rejects the notion of ‘malfunction’ as unscientific. This is the route taken by Wouters (1999, Section 7.3.3) and, more forcefully, Davies (2001). These authors argue that malfunction (in the sense of failure to perform a proper function) does not and should not have a place in the practice of biological science. If a certain organism does not circulate its blood due to a deviation of the heart, the problem is not that the heart fails to perform a function it should perform, but rather, that the organism does not perform a function that, given the way in which that organism is organized, needs to be performed. As Davies puts it “nonengineered, natural traits do not and cannot malfunction. As naturalists, we should not pretend otherwise” (Davies, 2001: 190). A difficulty related to both the ‘too liberal’ objection and the normativity issue is Millikan’s (1993a, 2002) problem that the systemic account lacks the means to make an appropriate function – accident distinction (intuition (1) and (5)) (Millikan calls this, somewhat confusingly, the indeterminateness problem). Cummins (1975) conceives of the function-accident distinction as a distinction between what is and what is not important to the capacities to be explained in an explanation by functional analysis. Pumping the blood is a function of the heart because this activity helps to explain the capacity to circulate the blood, producing a blublub sound is not a function of the heart because that activity does not figure in an explanation of an important capacity. However, surely, one does not want to attribute functions to all things that help to explain an important capacity, for this would lead one to attribute functions to things one normally counts as merely physical background conditions. Blood circulates, for example, only if gravity is within certain limits but we do not want to say that it is the function of the Earth’s gravity constant having a certain value to enable blood circulation. However, Cummins’s account seems to lack the resources to make a principled distinction between system and background conditions. As a first attempt one might want to limit the system of which the parts have functions to the body and count all other things as background conditions. However, if one counts all bodily activities that contribute to an important capacity as functions, one cannot make the distinction between a function and an accident, for any accidental contribution to survival is to count as a function. Cummins (1975) emphasizes that the explanatory role of function attributions is to explain a capacity to which the exercise of the function contributes, rather than the presence of the item to which the function is attributed. Millikan (1989a) agrees with Cummins that functions in his sense cannot be used to explain the presence of the item to which the function is attributed. However, according to her, there is another sense of function (namely function as selected effect) and attributions of functions in this sense intrinsically explain the presence of the item to which the function is attributed, because they single out for what effects the item was selected in the past. This view (the view that there are two kinds of functions, a systemic one that explains capacities resulting from the exercise of that function and an etiological one that explains the presence of to which this function is attributed) has become the dominant view among philosophers. Wouters (1999: chapter 5, 2005) argues against this view that biologists appeal to systemic functions (biological roles) not only in (1) explanations that explain an activity or capacity by specifying an underlying mechanism (Cummins’ explanations
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by functional analysis, biologists call such explanations ‘causal explanations’), but also in (2) explanations that explain the structure and activity of a part or behavior by appeal to its utility (biologists call such explanations ‘functional explanations’, Wouters calls them ‘design explanations’), and (3) explanations that explain the evolution of a trait by appeal to selection (evolutionary-historical explanations).
3.3 The Goal Contribution Approach Goal contribution theories define functions as causal contributions to the maintenance of a goal state. On this approach it is a function of the heart to pump blood, because pumping blood is what the heart does that contributes to the organism’s goal of survival and reproduction. The property of ‘having a goal’ is a cybernetic characteristic of a system, that does not imply consciousness or intentionality: a system has a goal if (within a certain range of boundary conditions) that system is disposed to vary its behavior in the manner required to achieve or maintain the goal. Given this definition of what it is to have a goal, living organisms clearly have the goal to survive and reproduce and biological functions are those functions in which this is the goal. The goal contribution approach is usually attributed to Nagel (1961). Its classic expositions are those of Boorse (1976) and Adams (1979). Boorse (2002) presents a brilliant response to most objections against the goal contribution theory. The basic difference between the goal contribution approach and the systemic approach is this: according to the goal contribution approach the appropriateness of function talk is the result of the way in which a certain system behaves (function talk is appropriate if a system shows persistence and flexibility in the maintenance of a certain state), according to the systemic approach the appropriateness of function talk is the result of the way in which a certain system is organized (function talk is appropriate if a system is complex and the behavior of the whole depends on the exact spatial relation of the parts and the timing of their activity). As the two features often go hand in hand, in many cases both kinds of theories will reach the same conclusion. The verdict may differ in those cases where the exercise of a complexly achieved activity does not contribute to or is detrimental to survival or reproduction, flight of small birds on stormy islands is a case in point (intuition 3). Systemic theories will depict such activities as functions, goal contribution theories will depict these activities as non-functional (at least at first sight). The goal contribution theory allows the attribution of functions to traits that do not vary (intuition 7), traits of instant organisms (intuition 8), traits of sterile organisms (intuition 12) and traits that are selected against (intuition 15). By identifying function with utility the goal contribution theory avoids the ‘too liberal’ objection. On the other hand, it excludes traits, items or behaviors that do not contribute to survival and reproduction from having functions. This runs against the counterexamples of intuition (3). More importantly, Amundson and Lauder (1994) point out, that biologists often investigate functions without considering the effects on the alleged goal of survival and reproduction. Schaffner (1993: 388, 399) and Boorse (2002) reject the intuition (3) that effects that are detrimental to survival can be functional. According to them, this idea is the result of the conflation of past and present functions and/or individual and species typical functions. The normativity problem and the indeterminateness problem discussed in connection with the systemic view apply equally to the goal contribution theory and can be answered
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in similar ways. Objections that are specific for the goal contribution theory are the criticisms of the cybernetic analysis offered by Nissen (1980) and Bedau (1992). In regard to indeterminateness, Boorse’s (2002: 81–83) response to Nissen (1997) is interesting. Nissen has raised a problem for the goal contribution theory that is analogous to Millikan’s indeterminateness problem. According to Boorse (1976) and Adams (1979) a thing can only have a function if it is part of a goal directed system. However, spider webs, bird nests, and rabbit burrows are neither part of an organism, nor of any other natural systems, yet, they have functions. Adams and Boorse solve this problem by referring to composite systems, such as the bird–nest system. Nissen objects that this is arbitrary. In addition it would allow one to attribute functions to inanimate objects, for instance it would be the function of an updraft to support an eagle and of rain to water the garden. Boorse (2002) responds by withdrawing the requirement that in order to have a function, an object must be part of a goal directed system. Things can have functions in relation to goal-directed systems of which they are not a part, if they regular contribute to the goal of that system. The consequence that many inanimate objects will have functions is acceptable as long as we are clear that such functions do not explain the existence, activity or location of that object. Like Wouters (1999) and Davies (2000), Boorse (2002) rejects the notion of malfunction. He argues that the idea that individual items can have functions that they cannot perform (intuition 11) rests on confusions between type and token, function and normal function and, in the case of artifacts, actual and intended function. If Carla’s heart cannot pump blood, then pumping blood is not, in fact, the function of her heart; it has no function. Since blood-pumping is the normal [i.e. species typical] function of a human heart, it would be the function of Carla’s heart if Carla’s heart pumped blood normally; but it does not, so it is not (Boorse, 2002: 89). It is, however, possible that item types have typically unperformed functions. The function of sperm cells to fertilize an egg cell is a case in point (intuition 12). This is because (most) functions are occasion specific and certain types of occasion are rare. An occasion specific function for which the occasion seldom arises is typical unperformed. Failure of a sperm to fertilize a non-existent ovum is like failure of blood to clot in a non-existent wound, or failure of sweat glands to release sweat when core body temperature is not above a certain level (Boorse, 2002: 93). Boorse (2002) claims that the goal contribution theory provides a unified account of artifact and biological function, both in physiological and in evolutionary contexts. Past goal contribution functions can help to explain a trait’s evolution in a straight-forward way.
3.4 The Life Chances Approach The life chances approach defines the functions of a trait as the effects of that trait that contribute to the life chances of its bearers being higher than the life chances of hypothetical organisms in which that trait is replaced by another one. On this account it is the function of the heart to pump blood because pumping blood is an effect of the heart and the life chances of organisms that have hearts are better if their heart pumps blood than if it does not. Life chances theories were proposed by Canfield (1964, 1965),
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Ruse (1971, 1973), Wimsatt (1972), Bigelow and Pargetter (1987) and Horan (1989). The main difference between this approach and the preceding two is its reliance on counterfactual comparison.5 Counterfactual comparison means that the actual organism is compared to a hypothetical one that need not exist, have existed or be able to exist. For example, according to life chances theories, to show that pumping the blood is a function of the heart, the actual organism (with a heart that pumps the blood) should be compared with a hypothetical organism that is similar to the real one except that the heart does not pump the blood. Because the life chances of the latter one would, clearly, be much lower than the life chances of the former, pumping blood is one of the functions of the heart. Bigelow and Pargetter (1987) argue that the life chances approach6 is to be preferred over etiological accounts on the ground that it does justice to the intuition that function attributions characterize a trait in terms of its future potential. As the etiological theory defines ‘function’ in terms of past selection, according to this theory and contrary to Bigelow and Pargetter’s intuitions, an effect that occurs for the first time is not yet a function and an effect that is no longer useful can still be a function. Parts and behaviors of instant organisms lack functions. On the life chances approach, however, traits have a function as soon as and as long as they enhance the propensity to survive, no matter their history. Traits that initially were neutral or detrimental to survival acquire a function as soon as they become fitness enhancing (for example in a new environment). A previously functional effect ceases to count as a function as soon as that effect no longer enhances fitness. Parts and behaviors of instant organisms clearly have functions.
5
Whereas I emphasize counterfactual comparison as the distinguishing character of the life chances approach, many others would point to the emphasis on survival and reproduction. Amundson and Lauder (1994: 444) put the theories of the life chances approach in the same category as those of the goal contribution approach and see the assumption that biological function is associated with survival and reproduction as that what distinguishes these theories from Cummins’ (1975) systemic theory. Boorse (2002: 64) seems to agree. Walsh and Ariew (1996), on the other hand, see the life chances theory of Bigelow and Pargetter (1987) as an evolutionary theory of function. This is a mistake precisely because of the counterfactual character of the comparison by which functions are to be established according to Bigelow and Pargetter. Evolutionary functions deal with the differences between actual competitors, not with hypothetical alternatives. Nissen (1997) makes the same mistake when he lumps the counterfactual analyses of Canfield, Ruse, and Wimsatt together with the evolutionary view of Neander as theories based on natural selection. Others (e.g. Kitcher 1993) see life chances functions as a special case of systemic functions: life chances functions are systemic functions in which the system to be analyzed is the organism as a whole and the capacity to be analyzed is something like survival, reproduction and/or fitness. This too seems a mistake as the life chances theory does not simply say that functions contribute to fitness, but that they contribute to fitness being higher than that of a hypothetical alternative. 6 Bigelow and Pargetter call their particular version of the life chances approach the propensity theory. The details of this theory are rather unclear, but the gist seems to be that a function is an effect due to which the propensity of an individual to survive in its natural habitat is greater than it would be if the effect were not produced.
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Wouters (1999, 2003) argues that a life chances notion of function plays an important role in physiology, functional morphology and behavioral biology, where it is often referred to as ‘function’, ‘utility’, ‘functional advantage’, ‘survival value’ or ‘adaptive value’. This notion is at the heart of the kind of reasoning that biologists call functional explanation (and that he calls ‘design explanation’). One main objection against the life chances approach says that it is impossible to determine what would happen if an organism did not have the traits it actually has (Frankfurt and Poole, 1966; Baublys, 1975; Millikan, 1989a, 1993a). How would this organism be built and how would it behave? What would it look like? How should its life chances be determined? Another important problem is that it is difficult to see how function statements of this kind can be explanatory of the presence of the trait to which the function is attributed (at least on a causal account of explanation). A trait can perform its function only when the trait is present, so the performance of the function can not causally explain the presence of that trait in its bearer. One solution would be to view life chances functions not as explanatory of the presence of the item or behavior to which the function is attributed, but rather as explanatory of some effect of that item of behavior, such as current fitness, the survival of the organisms that have that item or the continued existence of the trait in the population (e.g. Mitchell, 1993; Kitcher, 1993; Walsh, 1996). However, as Wouters (1999) shows by means of many examples, in the practice of biological research, life chances functions are actually used to explain the way in which organisms behave and are built.7 Another solution is suggested by Horan (1989) who attempts to account for the explanatory force of function attributions by means of the inferential theory of explanation (instead of the causal one). Obvious counterexamples against life chances theories of function are the habits to attribute functions to the parts of sterile organisms (intuition 12) and to the parts and activities that contribute to a mechanism that is no longer useful (intuition 3). Like the theories of the systemic and goal contribution approach, life chances theories have difficulty with the notion of malfunction (intuition 11).
3.5 The Etiological Approach The etiological approach defines a trait’s functions in terms of its etiology (e.g. its causal history): the functions of a trait are past effects of that trait that causally explain its current presence. For example, according to this account, it is the function of the heart to pump blood because pumping blood is what hearts did in the past that explains their current presence. Philosophers tend to mention Wright (1973) or Wright (1976) as the origin of the etiological approach. However, the idea that the term function should be used to refer to those effects of a trait that explain why that trait was selected in the past was originally put forward by the evolutionary biologist George Williams in his groundbreaking Adaptation and Natural Selection (1966). Williams’ proposal made no headway among biologists.
7
Another problem for theories that suggest that life chances functions explain population level effects is this: it is not clear what comparison with hypothetical organisms can say about the dynamics of the population.
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As far as I know Gould and Vrba (1982) are the only ones who explicitly accept Williams’ convention. Wright was indeed the first author who applied the term ‘etiological’ to this kind of analysis. It is, however, not clear that Wright’s analysis really is etiological. Wright defines ‘function’ as follows: The function of X is Z if and only if: (1) Z is a consequence (result) of X’s being there (2) X is there because it does (results in) Z Note that Wright’s second clause states that X is there because it does Z. This clause is deliberately vague because Wright wanted his analysis to apply both to traits of organisms (such as hearts) and to technical artifacts (such as knives). However, the application of Wright’s analysis to biological functions sounds like an unacceptable form of teleology. According to Darwin’s theory of evolution, the parts and processes of organisms are there not because of what they do, but rather because of what they actually did (in past ancestral populations). A strict etiological analysis replaces (2) by: (2 ) X is there because it did Z (in the past) But even in this strict etiological reading, Wright’s account faces many problems. Boorse (1975) presented several famous counterexamples, among which the gas leak that persists because the plumber who tried to repair the leak was poisoned by the gas.8 From a biological point of view the main problem is that Wright’s account fails to distinguish between the individual level (my heart is there because in the past it pumped the blood around) and the population level (humans have hearts because their ancestors’ hearts pumped the blood around). Neander (1980) presented an etiological account that solves these problems. It does so by explicitly appealing to natural selection. Etiological accounts that explicitly appeal to some form of selection are usually called “selected effect theories”. On Neander’s theory, the function of a certain occurrence of a trait is to produce the effects for which past occurrences of that trait were selected for by natural selection (see also Neander, 1991a).9
8
As the gas leak (X) is still there because it poisoned a plumber (Z), it is, on Wright’s account, a function of the gas leak to poison plumbers. This, however, seems counterintuitive. 9 Neander does not clarify the notion of trait selection, but I take it that ‘what a trait is selected for’ means ‘what variants having that trait were selected for over variants lacking that trait’. Note, however, that this way of explicating trait selection generates a counterexample. Suppose that in a population of rabbits a mutation occurs that changes both the color of the fur and the color of the eyes; the change in fur color makes the rabbits less suspicious to predators; the color of the eyes is neutral. The mutant spreads through the population as the result of selection for camouflage. In this case, variants having the new eye color were selected over variants lacking that color because of selection for better camouflage, so according to Neander’s theory camouflage would be the function of the new color of the eyes, which is against our intuitions.
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Note that a selected effect theory does not require that a trait currently performs its function, neither that it regularly performs its function. It is sufficient that in the past the trait produced a certain effect and that past performance explains its current presence. This makes it possible that items have a function, without actually performing it (intuition 10) or without being able to perform it (intuition 11). This makes the selected effect theory the only theory that provides a normative account of function. The explicit appeal to natural selection in Neander’s account makes it unsuitable outside the biological domain. Millikan (1984) provides a selected effects account that should apply to such diverse things as organs, behaviors, artifacts, customs, parts of organizations, words and grammatical forms. Godfrey-Smith (1994: 351) observes that many biologists tend to distinguish functional from evolutionary explanations. This poses a problem to the etiological theory as this theory sees functional explanations as a kind of evolutionary explanations. GodfreySmith proposes to accommodate this insight by defining functions as those effects that explain the maintenance of the trait by natural selection in the recent past (the ‘modern history theory’ of function). In his view, functional explanations are concerned with the maintenance of a trait in the recent past and evolutionary explanations with the origin of the trait. Buller (1998) distinguishes two versions of the etiological theory: the selected effects version which requires that the functional trait has been selected in the past and a weaker version which merely requires that in the past the functional trait contributed to the fitness of its bearers. He argues that the weaker one is to be preferred, both to the stronger variant of the etiological theory and to Kitcher’s (1993) non-historical selection theory. Schwartz (1999) poses a problem for Godfrey-Smith’s (1994) modern history variant of the etiological theory: according to this theory traits have a function only if they have recently been maintained by selection to perform this function. However, many traits that are assumed to have functions may have been maintained by other mechanisms than selection for that function, for example they may have been maintained due to lack of variation or by selection for other effects. Schwartz proposes a continuing usefulness account to deal with this situation. According to this account traits have a function if (i) the trait is there because, in the past, it has been selected for that function, and (ii) after this selection period it remained useful to the organisms that had it. It is not clear how utility is to be defined. Etiological theories of function are especially attractive to naturalistic philosophers of mind and language. As I explained in Section 2.2, these philosophers aim to account for mental and linguistic phenomena such as aboutness and misrepresentation in naturalistic terms. The favorite way is teleosemantics (e.g. Millikan, 1984, 1986, 1993b; Papineau, 1987; Dretske, 1988). Teleosemantic theories of content claim that what a thought or sentence is about, is determined by the function of the system that uses or produces that thought or sentence. Misrepresentation occurs if those systems malfunction. Etiological theories of function are attractive to naturalistic teleosemantics because etiological theories account for functions in terms of a well founded scientific theory, namely Darwin’s theory of evolution by natural selection, and because these theories offer a suitable notion of malfunction: a system malfunctions if it fails to do what it was selected for. Another feature of the etiological approach that is attractive to many proponents is its assumed solution to the problem of how functions explain (Salmon, 1989: 111–116,
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Neander, 1991b; Mitchell, 1995). As I said in Section 2.2, it is difficult to see how functions can be used in causal explanations of the presence of the trait to which that function is attributed. The etiological approach solves this problem because it identifies the functions of a trait not with that trait’s effects in its bearers, but with past effects of past occurrences of that trait in ancestral organisms. Past effects can explain current presence, of course and the etiological approach identifies functions precisely with those past effects that explain current presence. Wouters (1999, 2003) argues that this supposed solution rests on a misunderstanding: the kinds of explanations that biologists call functional explanations employ counterfactually defined functions rather than historical ones. The etiological theory offers no insight in that kind of explanation. Etiological theories have three main problems. First: etiological theories neither allow one to attribute functions to new traits that have not yet been selected for, nor to the parts of instant organisms (intuition 9). Moreover, if a trait acquires a new function, it does not have that function until is has been selected for having that effect. Second, the theory does not allow one to attribute functions to traits that are advantageous but have in the recent past been maintained by other mechanisms than selection, for instance because the trait does not vary anymore (intuition 7) or because the variants lacking that trait die for other reasons before the disadvantage becomes important (see Schwartz, 1999 for convincing examples). It also does not allow one to attributed functions to traits that have recently been selected against (intuition 15). Third, as Amundson and Lauder (1994) point out, it is very difficult to reconstruct past selection. This makes the attribution of historically defined functions a matter of mere speculation. Another problem is the application of the etiological theory to biology. No etiologist has ever provided a detailed discussion of a real example of the use of etiological functions in biology. Even simple references to the biological literature are lacking. On the other hand, Amundson and Lauder (1994), Kitcher (1993) and Wouters (1999, 2003) provide many examples of functional and evolutionary-historical explanations in biology in which historically defined functions play no role at all.
3.6 Non-Historical Selection Theories Kitcher (1993) and Walsh (1996) object against the etiological theory that many biologists think of the functions of a trait as the effects for which that trait is currently selected. To take this observation into account they stick to the definition of function in terms of selection, but they neither require that the selection took place in the past nor that the function causally explains current presence. In their theories past functions and current functions are special cases of a more general notion of function. This is possible because functions are relative to certain conditions. In Kitcher’s view functions are relative to time: according to him, the function of a trait at a certain time is the effect for which that trait is selected for at that time. In Walsh’s theory functions are relative to a selection regime: the function of a trait in a selection regime is the effect for which that trait is selected for in that selection regime).10 Current functions are functions relative
10
Actually, as Wouters (2003: 465–466) points out, Walsh’s account is ambiguous. Walsh sees functions both as the contribution to fitness in a certain selection regime and as the effects for which a trait is selected in that regime but these two notions are not
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to the present/the present selection regime; historical functions are functions relative to a certain past/a past selection regime. Non-historical selection theories (according to which a function is an effect due to which the relative fitness of its bearers is higher than that of competitors) differ from life chances theories (according to which a function is an effect that increases the life chances of its bearers relative to hypothetical variants) in two crucial (and related) respects. First, life chances theories define function at the individual level (a function increases the life chances of its bearers), whereas non-historical selection theories define function at the population level (a function is what a trait is selected for in a certain population). Second, life chances theories rely on counterfactual comparison, whereas non-historical selection theories compare the actual organisms with their actual competitors.11 Godfrey-Smith (1994) rejects current selection functions on the ground that they would deprive function attributions of their explanatory use: current selection cannot be the cause of current presence. Kitcher argues that function attributions based on current selection can serve to explain why a trait will continue to be present in the (near) future. Walsh argues that appeals to current function explain why individuals currently have the fitness they have. Non-historical selection theories have other merits and other problems than etiological ones. Like the etiological theories, non-historical selection theories define utility relative to actual existing competitors. This solves the problem of the indeterminateness of the reference situation that confronts the life chances approach. Millikan’s problem about the distinction between on the one hand a function and on the other a side effect (intuition 1) or useful accident (intuition 5) seems also solved. Belt buckles do not have the function to deflect bullets because presumably there is no genetic variation in the habit to wear belt buckles and, hence, no selection for that trait. Similarly, noses do not have the function to support glasses (intuition 4) because the variation in noses is not important for the ability to wear glasses. It should be noted, however, that, according to non-historical selection theories, it is not intrinsic to an item/activity what its functions are. The difference between function and side-effect/accident is determined by external factors such as the presence of competing variants, the other characteristics of the organism and the environment in which it lives. It has nothing to do with the reason why that effect is produced. This may not fit the intuitions of those who find the distinction between function and side-effect/useful accident important. Furthermore, like the other non-etiological theories, non-historical selection theories provide an account of function as something that an item or behavior does (in certain circumstances), rather than of what it is supposed or expected to do. Although pumping blood is, according to non-historical selection theories, a function of well-formed hearts (at least if there regularly turn up variants with deviant hearts and the variants with a well-formed heart have a greater relative fitness due to the fact that their heart is
co-extensive. In this paper I assume the latter interpretation (the first is a variant of the life-chances approach). 11 This difference is not always appreciated, both Kitcher (1993) and Walsh (1996), for example, see the life chances theory of Bigelow and Pargetter (1987) as a selection theory.
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not deviant), it cannot be the function of a malformed heart because variants with a malformed heart are not fitter than variants with a well-formed heart (or if they are it is not due to the malformation). In other words, non-historical selection theories do not provide a normative account of function, and, hence, they cannot serve the purposes of naturalistic philosophers of mind. Like life chances theories non-historical selection theories do not allow for maladapted functions (intuition 3). From a biological point of view non-historical selection theories have two other kinds of problems. First, determining current selection is very difficult (though perhaps not as difficult as determining past selection). In order to do so one needs information about (i) which variants occur in the population, (ii) the genetics of this variation, (iii) the phylogenetic relationships between the variants, (iv) the environments in which those organisms live, (v) the relative fitness of the different variants in those environments, (v) the relation between the differences between the variants and the fitness differences, and (vi) the structure of the population. There are many well-documented cases of selection (see for example, Endler, 1986), but these cases do not suffice to determine the selection forces currently affecting the evolution of hearts, lungs, livers, etc. To attribute current non-historical selection functions to these organs would be highly speculative. Furthermore, one might expect that the selection forces that influence the evolution of an organ differ among different species (think of the different requirements for fish hearts, crocodile hearts, bird hearts, leopard hearts and human hearts). This makes it difficult to attribute general functions such as ‘pumping blood’ to those organs. The second problem is that according to non-historical selection theories it is impossible to attribute functions to traits that do not vary (intuition 7) and traits that are selected against (intuition 15).
3.7 One Notion of Biological Function? Or More? It will be clear from the overview above that the different intuitions are difficult if not impossible to reconcile in one theory. This raises the question whether there is one theory that fits all purposes. Originally, the different philosophical accounts of biological function were seen as rival accounts of the unique meaning this notion was assumed to have. However, in the course of the 1990s the awareness grew that there are several senses of function involved in the study of biology. As I indicated in Section 2.4, nowadays many philosophers acknowledge two main notions of biological function: a teleological, evolutionary notion that serves to explain trait presence and a (non-evolutionary) physiological one that serves to explain complex activities or capacities (see for example Millikan, 1989a, 1993a, 2002; Godfrey-Smith, 1993; Kitcher, 1993; Griffiths, 1993; Amundson and Lauder, 1994, Walsh and Ariew, 1996; Walsh, 1996). Whereas Millikan and Godfrey-Smith suggest that these two notions are to be analyzed in different ways (along the lines of the selected effects and the systemic approach, respectively). Kitcher and Buller opt for a unifying account. Kitcher’s (1993) account is meant to apply both to artifact and to biological function. In its most general sense the function of an item is to do what it is designed for. The sources of design differ: in the case of artifact function the source is human intention; in the case of biological function it is natural selection. In both the artifact and the biological case the reference to the design process can be direct or indirect. The notion of function in physiology appeals to selection in an indirect way:
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the exercise of the function is assumed to help the organism to respond to a selection pressure, but it is not implied that the trait was selected for the performance of that function. Buller (1998) maintains that his weak version of the etiological theory (which appeals to past contribution to fitness rather than to past selection) applies both to the evolutionary notion and to the physiological notion. According to him this is the main reason why his weak etiological theory is superior to the selected effects theory. As I have indicated above (Section 2.4) I have doubts about the view that evolutionary studies in biology employ a historical and/or evolutionary notion of function. The selected effects theory fits the interests and intuitions of naturalistic philosophy of mind and language very well. However, it should not be pretended that this notion helps us to understand (evolutionary) biology. This is not to say, that biologists use the term ‘function’ unambiguously. The animal morphologists Bock and Von Wahlert (1965) observed that the term ‘function’ has two different meanings, namely ‘what it does’ and ‘what it is used for’. They propose to restrict the term ‘function’ to the first use and coin the expression ‘biological role’ for the second. Among biologists, Bock and Von Wahlert’s distinction is widely acknowledged, but their proposal to restrict the term function to ‘what it does’ found few followers (perhaps Mayr was the only one who adopted it). Among philosophers, the paper was largely ignored, until Amundson and Lauder (1994) argued (mistakenly, see Wouters, 1999: 117–119) that Bock and Von Wahlert’s (1965) notion of function is similar to that of Cummins (1975). I have argued elsewhere (Wouters, 1999, 2003) that the term ‘function’ is used by biologists in at least four different ways, namely to refer to (1) the activity of an item or of the organism as a whole (function in Bock and Von Wahlert’s (1965) sense), (2) the role of an item/activity in bringing about an activity of a mechanism of which that item/activity is a part (Bock and Von Wahlert’s (1965) notion of ‘biological role’), (3) the manner in which an item/activity contributes to the fitness of a certain organism being higher than that of a hypothetical organism with which it is compared (the ethologist’s notion of ‘function’, ‘survival value’ or ‘adaptive value’; I have called this ‘function as biological advantage’), (4) the effects for which a trait was selected in the past (Williams’ (1966) historical notion of function). The systemic approach and the goal contribution approach can be seen as rival attempts to analyze the notion of function as biological role; the life chances approach is concerned with the notion of function as biological advantage; and the etiological approach is concerned with the notion of function as selected effect.
4. CONCLUSION As I said in the introduction, the main area of philosophy in which biological notions of function are discussed is not the philosophy of biology, not even general philosophy of science, but philosophy of mind. An analysis of biological function is primarily seen as a preliminary to a reductionistic account of mental content and aboutness. This has several important consequences. One is that the focus is almost exclusively on the definition of function, whereas the problem of functional explanation is neglected. Second, because naturalistic philosophers of mind need a normative notion of function and teleology seems the only way to cash out normativity, there is a tendency to read much more teleology into biology than there really is. An indication is the tendency to talk of teleological explanations, where biologists would talk about functional explanations.
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Claims that those explanations are teleological in nature remain unsubstantiated, as does the assumption that norms play an important role in functional and comparative biology. Another problem is that the philosophical analysis of function and functional explanations heavily rests on intuitions, rather than on an analysis of what is going on in biology (even by those, like Millikan, who reject the conception of philosophy as conceptual analysis). In addition, folk tales like the one that chameleons adapt their pattern of skin color to the background and the one that lemmings migrate over large distances are presented as serious biology. Finally because selection is seen as the process that determines the purpose (and hence the norms), there is a tendency to overemphasize the importance of evolutionary biology. Time has come to liberate the study of biological function from the yoke of the philosophy of mind and study the use of the notion of function in actual biology. I do not deny that a stipulated definition of a teleological notion of function can solve many problems in the philosophy of mind and language. I do not see, however, how such a definition helps to understand biology. To understand biology one should study the actual role of function attributions and functional reasoning in actual biology. As I have argued elsewhere (Wouters, 1999, 2005), the main reason why biologists look for functions is not that they want to know which effects of an item or behavior are there on purpose and which by accident, but rather because they want to know how an item or behavior is used (see also Craver, 2001). The point of function attribution in biology is neither to distinguish functional effects from side-effects (intuition 1) nor to distinguish purposive effects from accidental ones (intuition 5) but rather to situate an item or behavior in the context of the organism as a whole. In doing so function attributions provide the handle to understand biological organization.
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