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Almost Identical, Almost Innocent

Published online by Cambridge University Press:  03 July 2018

Katherine Hawley*
University of St Andrews


In his 1991 book, Parts of Classes, David Lewis discusses the idea that composition is identity, alongside the idea that mereological overlap is a form of partial identity. But this notion of partial identity does nothing to help Lewis achieve his goals in that book. So why does he mention it? I explore and resolve this puzzle, by comparing Parts of Classes with Lewis's invocation of partial identity in his 1993 paper ‘Many But Almost One’, where he uses it to address Unger's problem of the many. I raise some concerns about this way of thinking of partial identity, but conclude that, for Lewis, it is an important defence against accusations of ontological profligacy.

Copyright © The Royal Institute of Philosophy and the contributors 2018 

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1 Lewis, David, Parts of Classes (Oxford, and Cambridge MA: Basil Blackwell Ltd., 1991), viiGoogle Scholar.

2 Lewis, David, ‘Mathematics is Megethology’, Philosophia Mathematica 1 (1993), 323CrossRefGoogle Scholar, at 3. Burgess, John P., ‘Lewis on Mereology and Set Theory’, in A Companion to David Lewis, (ed.) Loewer, Barry and Schaffer, Jonathan (Malden MA: Wiley-Blackwell, 2015)Google Scholar, is a user-friendly guide to Lewis's writing on these topics, and includes a fetching attempt to make ‘mereoplethynticology’ happen. Karen Bennett, ‘“Perfectly Understood, Unproblematic, and Certain” Lewis on Mereology’, in the same volume, explores many of the themes I touch on in the present article.

3 Lewis, Parts of Classes, 73–74.

4 Ibid., 81.

5 Cotnoir, A.J. and Baxter, D.L.M. (eds), Composition as Identity (Oxford: Oxford University Press 2014)CrossRefGoogle Scholar.

6 Katherine Hawley, ‘Ontological Innocence’, in Composition as Identity, 70–89.

7 Lewis, Parts of Classes, 82.

8 Baxter, Donald L.M., ‘Identity in the Loose and Popular Sense’, Mind 97 (1988), 375–82Google Scholar, and Baxter, Donald L.M., ‘Many-One Identity’, Philosophical Papers 17 (1988), 193216CrossRefGoogle Scholar.

9 Lewis, Parts of Classes, 82,

10 Armstrong, D.M., Universals and Scientific Realism II (Cambridge: Cambridge University Press, 1980), 3738Google Scholar.

11 Lewis, Parts of Classes, 73.

12 van Inwagen, Peter, ‘Composition as Identity’, Philosophical Perspectives 8 (1994), 207220CrossRefGoogle Scholar has illuminating remarks about Lewis's terminology here, in his footnote 2.

13 Van Inwagen, ‘Composition as Identity’, 214–5.

14 Ibid., 216.

15 Lewis, David, ‘Many, But Almost One’, pages references as republished in his Papers on Metaphysics and Epistemology (Cambridge: Cambridge University Press 1999)CrossRefGoogle Scholar.

16 Unger, Peter, ‘The Problem of the Many’, Midwest Studies in Philosophy 5 (1980), 411–67CrossRefGoogle Scholar.

17 Lewis, ‘Many But Almost One’, 176.

18 Armstrong, D.M., ‘Reply to Lewis’, in Ontology, Causality and Mind: Essays in Honour of D.M. Armstrong, edited by Bacon, John, Campbell, Keith and Reinhardt, Lloyd (Cambridge University Press, 1993): 3842Google Scholar. Bradley, F.H., ‘On Professor James' Doctrine of Simple Resemblance’, Mind 2 (1893), 8388CrossRefGoogle Scholar.

19 For examples, see Denkel, Arda, ‘Resemblance Cannot Be Partial Identity’, Philosophical Quarterly 48 (1998): 200204CrossRefGoogle Scholar, Baxter, D.L.M., ‘Instantiation as Partial Identity’, Australasian Journal of Philosophy 79/4 (2001): 449–64Google Scholar, and Mantegani, Nicholas, ‘Instantiation is Not Partial Identity’, Philosophical Studies 163/3 (2013), 697715CrossRefGoogle Scholar.

20 Lewis, ‘Many But Almost One’, 178.

21 Lewis, ‘Many But Almost One’, 178.

22 Lewis, ‘Elusive Knowledge’, Australasian Journal of Philosophy, 74, 549–67.

23 Schaffer, Jonathan, ‘Lewis on Knowledge Ascriptions’, in Loewer, and Schaffer, (eds), A Companion to David Lewis (Wiley-Blackwell 2015)Google Scholar is a helpful guide.

24 Lewis, ‘Many But Almost One’, 178.

25 Useful surveys of this material include Gilmore, Cody, ‘Location and Mereology’, Stanford Encylopedia of Philosophy (ed.) Zalta, E. (Fall 2017)Google Scholar and Kleinschmidt, ShievaIntroduction’ to Mereology and Location (ed.) Kleinschmidt, (Oxford University Press, 2014)CrossRefGoogle Scholar.

26 There may also be more complicated reasons to question the possibility of ordering by degree of mereological overlap, if gunk is actual; see Hudson, Hud, ‘Simples and Gunk’, Philosophy Compass 2.2 (2007) 291302CrossRefGoogle Scholar.

27 Lewis, ‘Many But Almost One’, 178–9.

28 This type of ontic indeterminacy does not lead inevitably to ontic indeterminacy in existence, as I discuss in ‘Vague Existence’, Proceedings of the Aristotelian Society CII (2001–2), 125–40.

29 Lewis, David, On the Plurality of Worlds (Basil Blackwell, 1986), 212–13Google Scholar.

30 Markosian, Ned, ‘Brutal Composition’, Philosophical Studies 92/3 (1998), 211249CrossRefGoogle Scholar.

31 Williamson, Timothy, Vagueness, (London: Routledge, 1994)Google Scholar.

32 Hawley, Katherine, ‘David Lewis on Persistence’, in Loewer, B. and Schaffer, J. (eds), A Companion to David Lewis (2015)Google Scholar.

33 As explored by e.g. Hawthorne, John, Metaphysical Essays (Oxford: Oxford University Press 2006)CrossRefGoogle Scholar, and Inman, Ross, ‘Neo-Aristotelian Plenitude’, Philosophical Studies 168/3 (2014), 583–97CrossRefGoogle Scholar.

34 I am grateful to Aaron Cotnoir for his helpful advice.