Full Idea
The doctrine that we can ask whether this is the same individual in another possible world, and that a common 'thisness' may underlie extreme dissimilarity, or distinct thisnesses may underlie great resemblance, I call 'Haecceitism'.
Gist of Idea
'Haecceitism' says that sameness or difference of individuals is independent of appearances
Source
David Kaplan (How to Russell a Frege-Church [1975], IV)
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.217
A Reaction
Penelope Mackie emphasises that this doctrine, that each thing is somehow individuated, is not the same as believing in actual haecceities, specific properties which achieve the individuating.