Full Idea
The extreme ontological alternatives with respect to the metaphysics of propositions are a Fregean Platonism (his "gedanken", 'thoughts'), and a radical nominalism or inscriptionalism, as in Quine, where they are just marks related to stimuli.
Gist of Idea
The extreme views on propositions are Frege's Platonism and Quine's extreme nominalism
Source
Dale Jacquette (Ontology [2002], Ch. 9)
Book Reference
Jacquette,Dale: 'Ontology' [Acumen 2002], p.221
A Reaction
Personally I would want something between the two - that propositions are brain events of a highly abstract kind. I say that introspection reveals pre-linguistic thoughts, which are propositions. A proposition is an intentional state.