Propositional attitudes, like believing and asserting, are relations between agents and propositions. Agents are individuals who do the believing and asserting; propositions are things that are believed and asserted. Propositional attitude ascriptions are sentences that ascribe propositional attitudes to agents. For example, a propositional attitude ascription α believes, or asserts, that S is true iff the referent of a bears the relation of believing, or asserting, to the proposition expressed by s. The questions I will address have to do with the precise nature of propositions, and the attitudes, like belief, that we bear to them.
I will assume both that propositions are the semantic contents of sentences, and that the proposition expressed by a sentence is a structured complex made up of the semantic contents of the parts of the sentences that express it.