Working at the ancestral home of relative identity, I feel the need to say something about it. The relative identity theorist says that we can’t speak of x and y being identical simpliciter but only identical relative to a sortal, and that while x and y might be the same F they might not be the same G. So, for example, while the foetus in 1979 might be the same biological organism as that typing this blog post, it is not, perhaps, the same person.
Something I was reading recently suggested the following argument against relative identity. The cases that are remotely plausible as being cases of relative identity are (like the one above) cases of identity across time. There are no plausible cases of relative identity at a time. But if that’s the case, we should think that identity at a time is absolute. In which case we shouldn’t think that the relation that is holding across time and is sortal relative is identity at all. And so we don’t really have relative identity: we’ve just got no cases of identity across time, but with a surrogate relation that holds between entities across time and is like identity in some respects but which is sortal relative.
I guess I think that’s a good argument against relative identity if the premise is correct, but I’m not convinced that there aren’t cases of relative identity at a time that are just as plausible as the cases of relative identity across time.
Before I give my case, consider the following situation. Suppose there is a sculptor, Bob. Bob takes a lump of clay at t1 – call it CLAY – and makes it into a statue of a man at t2 – call it STATUE. We have the familiar question as to the relationship between CLAY and STATUE. Some will say that STATUE is a proper temporal part of the CLAY, some that it is distinct but constituted by CLAY. The relative identity theorist, as I understand her, thinks she has a simpler story: STATUE is the same lump of clay as CLAY, but not the same statue (since CLAY is not a statue, and x and y are only the same F if they are both Fs). But that is relative identity across time, of course, not at a time. We don’t get relative identity at a time on this story: STATUE is both the same statue and the same lump as STATUE, and CLAY is the same lump as CLAY, and it doesn’t make sense to either say or deny that CLAY is the same statue as CLAY, since it isn’t a statue.
Now suppose another sculptor, Sara, made a different statue from a different lump. Sara’s sculpture is an intrinsic duplicate of Bob’s sculpture (STATUE), but while Bob’s sculpture is of a man, Sara’s sculpture is of a lump of clay shaped like a man. Sara’s sculpture has aesthetic properties that Bob’s sculpture lacks: her work is a comment on the very nature of art and representation. You can imagine her sipping Merlot out of a teacup and proclaiming the impossibility of separating the signifier from the signified, or something.
Now suppose there is a third sculptor, Jacob, who decided to kill two birds with the one stone and sculpt two statues from the one lump of clay: a statue of a man and a statue of a lump of clay shaped like a man. We know they are two, because they differ in their aesthetic properties. At a single time t after the sculpting is complete, then, there is the lump of clay, LUMP, the statue of a man, MAN, and the statue of the lump of clay shaped like a man, LUMP-MAN. MAN is the same lump of clay as LUMP, and LUMP-MAN is the same lump of clay as LUMP. It seems to follow that MAN is the same lump of clay as LUMP-MAN. Now, the logic of relative identity is somewhat up for grabs, but x is the same F as y and y is the same F as z seem to entail that x is the same F as z. Counterexamples to transitivity should only arise when there’s a change in sortal. And in any case, it’s independently plausible that MAN is the same lump of clay as LUMP-MAN, since there’s only one lump of clay in the vicinity. But MAN is not the same statue as LUMP-MAN – they are distinct statues, for they have different aesthetic properties. So MAN and LUMP-MAN are, at one time t, the same lump of clay but different statues. So we have relative identity at a time.
Now of course there are loads of things we could say without invoking relative identity, such as that the one lump constitutes two statues at this time, or (my preference) that you can’t conclude that there are two statues from a difference in aesthetic properties. But that’s not the point. There’s always an absolutist story one can tell when the relativist would tell a relativist story; my claim is only that this case of relative identity at a time is just as plausible as the alleged cases of relative identity across time, in which case the above argument against relative identity is unsound, and doesn’t give us reason to accept absolutism. (Common sense on the other hand . . . )