Full Idea
The difficulty is that Kripke individuates objects by their modal properties, by what they (essentially) could and could not be. Kripke's ontology presupposes essentialism; it can not be used to ground it.
Gist of Idea
Kripke individuates objects by essential modal properties (and presupposes essentialism)
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by Hilary Putnam - Why there isn't a ready-made world 'Essences'
Book Reference
Putnam,Hilary: 'Realism and Reason: Papers vol 3' [CUP 1985], p.220