Full Idea
Adams favours haecceitism about actual things but no haecceities for things that might exist but don't.
Gist of Idea
Adams says actual things have haecceities, but not things that only might exist
Source
report of Robert Merrihew Adams (Actualism and Thisness [1981]) by Robert C. Stalnaker - Mere Possibilities 4.2
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.97
A Reaction
This contrasts with Plantinga, who proposes necessary essences for everything, even for what might exist. Plantinga sounds crazy to me, Adams merely interesting but not too plausible.
Related Idea
Idea 16469 Plantinga has domains of sets of essences, variables denoting essences, and predicates as functions [Plantinga, by Stalnaker]