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Full Idea
Identity across possible worlds makes sense only with respect to a sortal
Clarification
A 'sortal' says what sort of thing it is
Gist of Idea
Possible worlds identity needs a sortal
Source
Allan Gibbard (Contingent Identity [1975], IV)
Book Ref
-: 'Journal of Symbolic Logic' [-], p.198
A Reaction
See Gibbard's other ideas from this paper. I fear that the sortal invoked is too uncertain and slippery to do any useful job, and I can't see any principled difficulty with naming something before you can think of a sortal for it.