- An intrinsic property, as David Lewis puts it, is a property "which things have in virtue of the way they themselves are", as opposed to an extrinsic property, which things have "in virtue of their relations or lack of relations to other things". Having long hair is an intrinsic property; having a long-haired brother is not. Intuitive as this notion is (and valuable in doing philosophy, I might add), it seems to resist analysis. Analysis, that is, to "quasi-logical" notions such as necessity, spatiotemporal location: using stronger tools, Lewis has given an analysis of intrinsicality that I take to be roughly correct.
- Lewis initially described intrinsic properties in his 1983 paper "Lewis (David) - Extrinsic Properties" as follows: If something has an intrinsic property then so does any perfect duplicate1 of that thing ... Later, in "Lewis (David) - On the Plurality of Worlds", he elaborated: ... two things are duplicates2 iff (1) they have exactly the same perfectly natural properties, and (2) their parts can be put into correspondence in such a way that corresponding parts have exactly the same perfectly natural properties, and stand in the same perfectly natural relations ... an intrinsic property is one that can never differ between two duplicates3.
- In giving these definitions, Lewis is assuming certain of his controversial views (although the definitions may be reformulated for other frameworks). The first is his modal4 realism; the import of this is that the "two duplicates5" mentioned at the end of the quotation may be non-actual objects, and they may come from different possible worlds. Moreover, they are assumed to be world-bound objects, and thus have their properties absolutely, and not relative to a world. Finally, Lewis assumes the metaphysics of temporal parts; the import of this is that he does not take property instantiation as being relative to a time; thus, a property that we might ordinarily ascribe to a continuant at a certain time will, for Lewis, be a property of a time-slice of such an object.
- For the moment, I will follow Lewis in making these assumptions; thus, Lewis's definition of 'intrinsic' may be stated as follows: (I) Property P is intrinsic iff for any possible objects x and y, if x and y are duplicates6 then x has P iff y has P.
- Thus, Lewis defines 'intrinsic' in terms of 'duplicate7', and 'duplicate8' in terms of 'perfectly natural'. As for 'perfectly natural', Lewis is neutral as to whether it is to be taken as primitive, or analyzed in terms of some other strong extra-logical primitive notion, such as that of an immanent universal, that of a trope, or some complex concept of similarity.
- I aim to defend this project. I think that (I) is a successful analysis of an important notion of intrinsicality, and moreover, that Lewis's use of the strong primitive of naturalness (or one of the other strong, extra-logical primitives) is no accident, for analyses in terms of weaker notions invariably fail.
- In section 1 I single out the target notion of intrinsicality, and then in section 2 I defend (I) as an analysis of that notion against objections due to J. Michael Dunn. Finally, I criticize other analyses of intrinsicality and related notions; in particular, I claim that no analysis of these concepts that proceeds purely in terms of "quasi-logical" notions is possible. (In doing so I will argue that there is no quasi-logical analysis of Lewis's notion of a perfectly natural property.)
- Thus, Lewis's use of the strong primitive notion of a natural property will be vindicated.
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