Beliefs, desires, perceptions, and other mental states and events are said to possess content. We attribute such states and events with sentences such as

(1) Arabella believes that the cat is crying.

(1) contains a propositional attitude verb ("believes") and a sentence complement ("the cat is crying"). The verb specifies a type of mental state (belief), and the complement sentence indicates the content of the state. On most accounts this content is the proposition expressed by that sentence. Propositions have been variously conceived as abstract entities composed of modes of presentation, sets of possible worlds, sets of synonymous sentences, and structured entities containing individuals and properties. All these accounts agree that propositions determine truth conditions. Some mental states and events (e.g., desiring to visit Paris) seem to have contents that are not propositions. However, for most of the current discussion, contents will be identified with propositions and...