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Full Idea
When pressed, philosophers will describe propositions as states of affairs or sets of possible worlds. But wait! Neither sets of possible worlds nor states of affairs - electrons being negatively charged, for instance - have truth values.
Gist of Idea
If propositions are states of affairs or sets of possible worlds, these lack truth values
Source
John Heil (From an Ontological Point of View [2003], Intro)
Book Ref
Heil,John: 'From an Ontological Point of View' [OUP 2005], p.10
A Reaction
I'm not sure that I see a problem. A pure proposition, expressed as, say "there is a giraffe on the roof" only acquires a truth value at the point where you assert it or believe it. There IS a possible world where there is a giraffe on the roof.