Full Idea
Someone who believes propositions are concrete cannot agree that some propositions are necessary. For propositions are contingent beings, and could have failed to exist. But if they fail to exist, then they fail to be true.
Gist of Idea
If propositions are concrete they don't have to exist, and so they can't be necessary truths
Source
Alvin Plantinga (Why Propositions cannot be concrete [1993], p.230)
Book Reference
Plantinga,Alvin: 'Essays in the Metaphysics of Modality' [OUP 2003], p.230
A Reaction
[compressed] He implies the actual existence of an infinity of trivial, boring or ridiculous necessary truths. I suspect that he is just confusing a thought with its content. Or we might just treat necessary propositions as hypothetical.