I have been going through Jeff King's book The Nature and Structure of Content in my metaphysics seminar and wanted to put down some thoughts (time permitting).
Very generally, King's account of structured propositions is as follows. Propositions are certain kinds of facts whose constituents are structured in the same way as the sentences which express them are structured at LF. Consider, for instance, the proposition that Rebecca swims. On King's account, this proposition is analysed as:
The fact of there being a (possible) context c and lexical items a and b of some language L such that a has as its semantic value in c Rebecca and occurs at the left terminal node of the sentential relation R that in L encodes the instantiation function and b occurs at R's right terminal node and has as its semantic value in c the property of swimming.
It is clear from reading the book (though the issue is not addressed directly) that King intends his language variable to range over languages-in-use (rather than, for example languages as abstract objects). Consequently, on King's view languages are mind-dependent entities which do not exist prior to the development of language.
This gives rise to the following worry. On the most widely accepted views of what it is for a language to be the language of a given community, this will involve various highly complex propositional attitudes (intentions, beliefs, etc.) on the part of the members of that community. But it seems clear that one cannot have those propositional attitudes unless, well, there are propositions. So it appears that King is commited to the claim that there must be propositions prior to language. But this is apparently inconsistent with his view of propositions.
King notes that he has two options (i) adopt the Language of Thought Hypothesis or (ii) appeal to protobeliefs/protointentions. The second option is important to King because he doesn't want to hang is hat entirely on the LOT hypothesis. But I am doubtful that this is a genuine option for him. Call the "contents" of the protobeliefs/intentions "protopropositions". Whatever else we want to say about protopropositions, it seems clear that if they are going to do the work that King wants they had better be both truth-evaluable and structured. But these are two of the primary characteristics of propositions which King's theory was supposed to capture. If he is now commited to giving a theory of structured protopropositions with these characteristics, it is likely that whatever theory is put in place can be adapted to handle propositions directly. If that is right, then King is more heavily committed to LOT than his discussion in the book suggests.