 Third, they are intrinsic to the particulars that instantiate them—which, all too roughly, means that they characterize what those particulars are like, independently of what any other distinct particular is like. More: The intrinsic nature of any particular is exhausted by what perfectly natural properties it  instantiates.[4]  This assumption also allows a theory of natural properties and relations to yield, in a fairly simple way, a definition of “perfect duplicate” applicable to any possible objects x and y (not necessarily inhabiting the same possible world): x and y are perfect duplicates iff they share exactly the same perfectly natural  properties.[5]  A definition of “intrinsic” follows: a property P is intrinsic iff any two duplicates x and y (taken from any possible worlds) either both have P or both fail to have P. Of course, what we really have here is a tight circle that puts on display how the expressions “intrinsic”, “perfect duplicate”, and “perfectly natural” can be interdefined, with the help of the modal notion of metaphysical possibility. (See Lewis 1983c and Langton &amp; Lewis 1998 for discussion of various strategies for breaking out of this circle.)