Trans-Index-Identity and Hyper-Essentialism (Again)
I need to tidy up this part of my belief space. Once I complained that literal trans-world identity (as opposed to trans-world identity based on similarity) is implausible because it entails that there can be no vagueness about a thing's essential properties (for determinate properties): either the thing has the property at all worlds or not. On the other hand, I also believe that there is no big difference between individuating things as worldbound and individuating them as trans-world fusions of worldbound counterparts. Unfortunately, these two views can't both be correct.
Whereas a counterpart relation may be gradual, the parthood relation isn't. So if I am the trans-world fusion of my counterparts, any thing at any world either definitely is or is not a part of that fusion. Hence if literal trans-world identity leads to hyper-essentialism then so does trans-world fusionism.
The obvious answer is that vagueness about essential properties arises because the linguistic and non-linguistic facts don't settle just which trans-world fusion I am. There are several candidates, differing in which borderline counterparts they include, and none of them is really me or not me. This is exactly the answer I always use to explain vagueness of temporal extension: when it it vague whether, say, some fetus is me, that's because there are various candidate trans-time fusions differing in whether they include the fetus or not as a temporal part, and it is indeterminate which of them is me.
This answer is open to the literal trans-world identitist as well. He can say that it is indeterminate whether Kripke essentially weighs more than 10 g because there are many candidate referents for our name "Kripke" some of which are strictly identical to other-worldly persons heavier than 10 g and some of which aren't.
There still is an advantage of the fusion answer over the identity answer: I can say that all the Kripke candidates completely overlap at our world, which explains why we don't think that there are many Kripkes at our world. On the literal trans-world identity theory, it is hard to see how this could be true given that "overlap" means what it means in standard extensional mereology. The same advantage favours the fusion view in the temporal case: I can say that all the Kripke candidates overlap at the current time. However, they do not overlap completely, that is, share absolutely all their parts, as otherwise they'd be identical (by mereology). So they differ in some of their parts. But these parts can only be temporal parts. And a believer in literal trans-time identity wants to reject those. Similarly, a believer in literal trans-world identity wouldn't like the modal parts that prevent the overlapping Kripke candidates from being identical, for these modal parts are just the (non-identical) counterparts he rejects.