Full Idea
Nothing can be predicated of something which does not exist.
Gist of Idea
Predicates can't apply to what doesn't exist
Source
Robert C. Stalnaker (Merely Possible Propositions [2010], p.28)
Book Reference
'Modality', ed/tr. Hale,B/Hoffman,A [OUP 2010], p.28
A Reaction
[He says he is 'agreeing with Plantinga' on this] This seems very puzzling, as you can obviously say that dragons do not exist, but they breathe fire. Why can't you attach predicates to hypothetical objects?