Why Propositions Can't Be Sets of Truth-Supporting Circumstances

It might be of interest to note that Soames has very recently posted a short paper responding to a criticism of his 'Direct Reference, Propositional Attitudes, and Semantic Content' that we read for last week. The new paper contains a summary of the main argument in the paper we read. So it might be useful as an overview of what Soames was up to in the (1987) paper.