Full Idea
If propositions are brain inscriptions, then if there had been no human beings there would have been no propositions. But then 'there are no human beings' would have been true, so there would have been at least one truth (and thus one proposition).
Gist of Idea
Propositions can't just be in brains, because 'there are no human beings' might be true
Source
Alvin Plantinga (Why Propositions cannot be concrete [1993], p.229)
Book Reference
Plantinga,Alvin: 'Essays in the Metaphysics of Modality' [OUP 2003], p.229
A Reaction
This would make 'there are no x's' true for any value of x apart from actual objects, which implies an infinity of propositions. Does Plantinga really believe that these all exist? He may be confusing propositions with facts.