An Argument for Counterpart Theory

[I have no idea if the following is at all novel or plausible.  Any feedback would be sweet!]

Here’s a puzzle.  David Lewis (1986) has argued for the following thesis:

L. Self-identity is not constituted, even in part, by having certain qualities.

Kit Fine (1994) argued for the following thesis:

F. An essential property of an object is any property that, in part, constitutes what it is to be that object.

Combining these two theses would seem to imply the following somewhat troubling thesis:

T. Objects do not have any qualities essentially.

I say that this thesis is troubling because, after all, it would seem to be part of, say, my essence that I have the quality of being human.[1] But how can it be both that I have no essential qualities and that being human is part of my essence? Let’s assume for the moment that we don’t want to reject either Lewis’s thesis or Fine’s thesis (I for one have been convinced by both authors). How then might we get out of trouble?

We might start by denying the claim that being human is part of my essence. We might try saying that although I am necessarily human, Fine (1994) has convinced us that not all necessarily had properties are essential properties,[2] and so it does not follow that I am essentially human. However, the claim that I am not essentially human just seems to me patently false: even if I accept Fine’s understanding of an essential property (which I do) as part of what it is for an object to be that object, it still seems to me that being human is part of my essence—that is, it seems to me that being human is part of what it is to be me.

The other way to avoid trouble is to deny the inference from (L) and (F) to (T). Is this plausible? At first it seems not. After all, the following seem to be straightforward translations of (L), (F), and (T)

L*. Given an object A and some x such that x = A, there is no quality Q such that x’s having Q constitutes, even in part, the fact that x = A.

F*. Given an object A and some x such that x = A, an essential property of A is any property P such that the fact that x has P constitutes, in part, the fact that x = A.

T*. Given an object A, there is no quality Q such that Q is an essential property of A.

And the inference from (L*) and (F*) to (T*) is clearly valid.

So what are we to do? Here’s one response I’d like to suggest: resist the interpretation of (F) as (F*) and instead hold that (F) should be understood as

F^C. Given an object A and x such that x is the counterpart of A, an essential property of A is any property P such that the fact that x has P constitutes, in part, the fact that x is the counterpart of A.

Of course, it’s plausible to interpret (F) as (F^C) only if our discourse is to be generally understood as about counterparts rather than interworld identicals. So in this way we have something of an argument for counterpart theory: it allows us to explain why three seemingly plausible theses are consistent, when otherwise they would be inconsistent.