Diamond
Enthusiast
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No. These words are never interchangeable. In fact, they are mutually exclusive. A proposition is, most simply stated, the cognitive content of some declarative sentence in some language. Think of Lincoln's phrase: the proposition that all men are created equal.
The proposition here is contained in and expressed by the sentence following that. In no way is this anything like a proposal of Lincoln's.
Proposals are recommendations for a certain course of action. Although they are often introduced in English in sentences preceded by the complementizer that, just as propositions are, the syntax is still different in some ways. If Lincoln had been making a proposal at Gettysburg, he would have had to say something like (I propose) that all men be created equal, which he quite wisely did not say (since the creation of all men, such as they are, was already an accomplished fact.)
You can entertain or consider both proposals and propositions. But proposals are made, but except in some off-color sense irrelevant here, propositions are not.
In logic, propositions are either true or false (or, according to some, perhaps undecidable). The terms true and false simply fail to apply to proposals, at least in the primary senses of these words (it is no counter-argument, in other words, to cite examples such as Nixon's true proposal was otherwise. Here, the word true is not being used in its primary sense.)
Finally, English verbs of the sort which express propositional attitudes (chiefly believe and know), take clausal complements expressing propositions, but never proposals. Indeed, English sentences of the form "I know that S" or "I believe that S" are uninterpretable where S is taken to be a proposal. We cannot say "I believe your proposal" or "I know your proposal to be true, even though we can say, with clearly different meanings of the verbs, "I believe inyour proposal" (i.e., I endorse it myself) or "I know your proposal" (i.e., I've heard of it).
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