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A Taxonomy of Functions

Published online by Cambridge University Press:  01 January 2020

Denis M. Walsh
University of Wisconsin, 600 North Park Street, Madison, WI53706, USA
André Ariew
University of Arizona, Tucson, AZ85721, USA


There are two general approaches to characterising biological functions. One originates with Cummins. According to this approach, the function of a part of a system is just its causal contribution to some specified activity of the system. Call this the ‘C-function’ (or ‘Cummins function’) concept. The other approach ties the function of a trait to some aspect of its evolutionary significance. Call this the ‘E-function’ (or ‘evolutionary function’) concept. According to the latter view, a trait's function is determined by the forces of natural selection. The C-function and E-function concepts are clearly quite different, but there is an important relation between them which heretofore has gone unnoticed. The purpose of this paper is to outline that relation.

This is not the first paper to discuss the relation of C-function and E-function. Previous attempts all follow either one of two strategies. The first proposes that the two concepts are ‘unified.’ The other proposes that they are radically distinct and apply to wholly different fields within biology.

Research Article
Copyright © The Authors 1996

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2 Cummins, R.Functional Analysis,Journal of Philosophy 72 (1975) 741–65CrossRefGoogle Scholar. Reprinted in Sober, E. ed., Conceptual Issues in Evolutionary Biology 2nd ed. (Cambridge, MA: The MIT Press 1994)Google Scholar. All quotations from this paper will be taken from the Sober reprint.

3 Kitcher, P.Function and Design,’ in French, P.A. Uehling, T.E. and Wettstein, H.K. eds., Midwest Studies in Philosophy XVIII (Minneapolis: University of Minnesota 1993) 379–97Google Scholar

4 Amundson, R. and Lauder, G.Function without Purpose: The Uses of Causal Role Function in Evolutionary Biology,Biology and Philosophy 9 (1994) 443–69CrossRefGoogle Scholar; Godfrey-Smith, P.Functions: Consensus Without Unity,Pacific Philosophical Quarterly 74 (1993) 196208CrossRefGoogle Scholar

5 In The Nature of Psychological Explanation (Cambridge, MA: The MIT Press 1983), Cummins amends his account of function to allow that s and x may also range over processes themselves (e.g. multiplying 27 by 32) and their component processes (e.g. mutliplying 2 by 7, adding 5 and 1), irrespective of how these capacities are instantiated.

6 It will become apparent that there is some dispute over how to interpret the question of why a feature is present. Some (Enç, B.Function Attributions and Functional Explanation,Philosophy of Science 46 [1979] 343–65CrossRefGoogle Scholar; and we) believe that function ascription yields genuinely teleological explanations of the presence of a feature (i.e. it explains why a feature persists by citing its effects); others (Neander, K.The Teleological Notion of “Function,”Australasian Journal of Philosophy 69 [1991] 454–68CrossRefGoogle Scholar; P. Kitcher, ‘Function and Design’; Godfrey-Smith, P.A Modern History Theory of Functions,Nous 28 [1994] 344–62CrossRefGoogle Scholar; Millikan, R.G.In Defense of Proper Functions,Philosophy of Science 56 [1994] 288302CrossRefGoogle Scholar) believe that function ascriptions merely explain the current presence of a trait by citing its causal history.

7 Wright, L.Functions,Philosophical Review 82 (1973) 139–68CrossRefGoogle Scholar

8 Mills, S. and Beatty, J.The Propensity Interpretation of Fitness,Philosophy of Science 46 (1979) 263–86CrossRefGoogle Scholar; see also Sober's Conceptual Issues in Evolutionary Biology.

9 Millikan, R.G. Language, Thought, and other Biological Categories (Cambridge, MA: The MIT Press 1984)Google Scholar and ‘In Defense of Proper Functions’; Neander, K.Functions as Selected Effects: The Conceptual Analyst's Defense,Philosophy of Science 58 (1991) 168–84CrossRefGoogle Scholar and ‘The Teleological Notion fo “‘Function’”; P. Godfrey-Smith,’ A Modern History Theory of Functions'

10 We shall use the following convention to distinguish trait types from tokens. Upper case italics ‘X’ are variables ranging over trait types; lower case italics ‘x’ are variables ranging over tokens, such that x is a token of X.

11 This is the reason for the ‘aetiological’ epithet.

12 Liem, K.Evolutionary Strategies and Morphological Innovations: Cichlid Pharyngeal Jaws,Systematic Zoology 22 (1973) 42.5-41CrossRefGoogle Scholar

13 We thank Farish A. Jenkins Jr. of the Museum of Comparative Zoology and Harvard Medical School for corroborating this.

14 See Bigelow, J. and Pargetter, R.Functions,Journal of Philosophy 84 (1987) 181–97CrossRefGoogle Scholar; Enç, B. and Adams, F.Functions and Goal Directedness,Philosophy of Science 59 (1992) 636–54CrossRefGoogle Scholar; Sober, E. Philosophy of Biology (Boulder, CO: Westview 1993)Google Scholar.

15 Mitchell, S.Dispositions or Etiologies? A Comment on Bigelow and Pargetter,Journal of Philosophy 90 (1993) 249–59CrossRefGoogle Scholar and ‘Function, Fitness, and Disposition,’ Biology and Philosophy 10 (1995) 39-54; P. Godfrey-Smith, ‘A Modem History Theory of Functions’

16 Enç and Adams (‘Functions and Goal Directedness’) also point out that functional explanations may be either forward-looking or aetiological, pace Mitchell, who contends that aetiology is the only explanatory project for which functions are invoked.

17 This is something that advocates of the historical theory readily admit (see especially Mitchell, ‘Dispositions or Etiologies?’). They tend to deny that expected persistence, or current fitness differences are any part of the explanatory applications of function in evolutionary biology.

18 See Millikan, R.G.Truth Rules, Hoverflies, and the Kripke-Wittgenstein Paradox,’ in White Queen Psychology and other Essays for Alice (Cambridge, MA: The MIT Press 1993)Google Scholar, esp. n. 10; and K. Neander, ‘Functions as Selected Effects.’

19 Although there are some problems with the formulation that Bigelow and Pargetter give to novel functions. See D.M. Walsh, ‘Fitness and Function,’ The British Journal for the Philosophy of Science (forthcoming) for a discussion.

20 Brandon, R. Adaptation and Environment (Princeton: Princeton University Press 1990)Google Scholar; Richardson, R.C. and Burian, R.M.A Defense of the Propensity Interpretations of Fitness,Philosophy of Science Association 1 (1992) 349–62Google Scholar

21 By ‘selective regime’ of a trait we mean the total set of abiological and biological (including social, developmental, and physiological) factors in the environment of the trait which potentially affect the fitness of individuals with that trait.

22 Walsh, ‘Fitness and Function’

23 See Sober, E. The Nature of Selection (Cambridge, MA: The MIT Press 1984)Google Scholar and Philosophy of Biology for the significance of selection for a trait.

24 A single regime, R, may be both historical and current; when that is so, the function is both historical and current.

25 Millikan, R.G.Propensities, Exaptations and the Brain,’ in White Queen Psychology and other Essays for Alice (Cambridge, MA: The MIT Press 1993), 13Google Scholar; emphasis in original

26 Including K. Neander, ‘The Teleological Notion of “Function”’; P. Kitcher, ‘Function and Design’; P. Godfrey-Smith, ‘Functions: Consensus without Unity’; R.G. Millikan, White Queen Psychology; Griffiths, P.E.Functional Analysis and Proper Functions,British Journal for the Philosophy of Science 44 (1993) 409–22CrossRefGoogle Scholar; and R. Amundson and G. Lauder, ‘Function Without Purpose.’

27 See Allen, C. and Bekoff, M.Biological Function, Adaptation, and Natural Design’, Philosophy of Science 62 (1995) 609–22CrossRefGoogle Scholar. There may well be an ahistorical sense of ‘design,’ but there is no ahistorical sense of ‘design by natural selection.’

28 This point is stressed by Godfrey-Smith in ‘Functions.’

29 We return to this question once we have developed our account of the C-function/E-function relation.

30 See R.G. Millikan, Language, Thought, and other Biological Categories ch. 1 and ‘In Defense of Proper Functions’; K. Neander, ‘Functions as Selected Effects.’

31 Cummins (in a personal communication) made this point to us quite vividly. According to his (C-function) conception, ‘the function of hearts explains circulation, not hearts.’ E-function explanations, in contrast, are natural selection explanations. On what natural selection explains, see Sober, E.Natural Selection and Distributive Explanation: A Reply to Neander,British Journal for the Philosophy of Science 46 (1995) 384–97CrossRefGoogle Scholar.

32 Junk DNA has an evolutionary function at the level of the gene. The E-function of junk DNA is to replicate itself. Replicating itself and having no fitness consequences for the individual has high fitness consequences for the junk DNA. Doing just that explains why junk DNA is present. These are the E-functions of junk DNA at the level of the gene.

33 Harvey may not have known he was discovering the E-function of the heart. Presumably he possessed neither the concept of fitness nor that of a selective regime. Nevertheless, he discovered (de re, so to speak) the E-function of the heart because he performed the kind of functional analysis by which E-functions are revealed.

34 A similar point is made by P.E. Griffiths in ‘Functional Analysis and Proper Functions.’ For a superb example of how E-function is revealed by C-function analysis, see Kingsolver and Koehl's discussion of the function and evolution of insect wings: Kingsolver, J.G. and Koehl, M.A.R.Aerodynamics, Thermoregulation, and the Evolution of Insect Wings: Differential Scaling and Evolutionary Change,Evolution 39 (1985) 488504CrossRefGoogle Scholar.

35 The functional analysis need not involve the token x itself.

36 Dennett, D.C.Intentional Systems in Cognitive Ethology: The “Panglossian Paradigm” Defended, in The Intentional Stance (Cambridge, MA: The MIT Press 1987) 237–86Google Scholar, at 278; italics in original. We thank an anonymous referee from this journal for raising this challenge.

37 We happen to think, contra Millikan and Neander, that traits are not individuated by their E-functions. For biologists, trait types are defined by relations of homology. Lauder, for example, discusses the use of functions in cladogram construction (Lauder, G. V.Homology, Form, and Function,’ in Hall, B.K. ed., Homology: The Hierarchical Basis of Comparative Biology (San Diego: Academic Press 1994) 151–96)Google Scholar. He concludes that sameness of function is evidence for — albeit defeasible evidence for — sameness of biological trait type. If sameness of function is defeasible evidence for sameness of biological trait type, then functions cannot be the criterion for individuating biological traits.

38 See also Cummins's, The Nature of Psychological Explanation, 29Google Scholar

39 Our claim here is reminiscent of a point made by Cummins. Arguing against a definition of function proposed by Nagel, Cummins says ‘it would at most tell us which effects are picked out as functions; it would provide no hint as to why these effects are picked out as functions (‘Functional Analysis,’ 60; emphasis in original). This echoes our criticism of Amundson and Lauder's view quite nicely. They correctly identify which C-functions are attributed by anatomists and physiologists, but do not answer the question why these effects are picked out as functions.

40 These, we believe, are the functions most commonly ascribed in comparative anatomy and physiology, those functions discussed at length by Amundson and Lauder in ‘Function without Purpose.’

41 We expect examples of this sort to be rare.