Full Idea
Plantinga says for an individual to exist with certain properties in some possible world is simply for it to be true that, had that possible world obtained, that individual would have existed with those properties.
Gist of Idea
Possibilities for an individual can only refer to that individual, in some possible world
Source
report of Alvin Plantinga (The Nature of Necessity [1974]) by Penelope Mackie - How Things Might Have Been 5.1
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.80
A Reaction
This is intended to dissolve the problem of transworld identity, and is certainly a flat rejection of counterparts. I take the point to be that the individual is the key element in defining the possible world, so can't possibly be different.