What is it to say we possess a mental state, to say that, say, we ‘have’ a belief, or that we ‘hold’ an opinion. Our language seems to suggest that we are some sort of vessel in which beliefs and opinions and other mental states may be deposited, and where these states may be inspected and seen to exist. But how? Other than introspection – which is notoriously unreliable, not to mention epistemologically suspect – how? Ascription theory says, basically, that for a person to ‘have’ a belief P is to say that (under typical conditions) this person would be ascribed by others (in empirically verifiable ways, such as observable statements and the like) to have that mental state. It is one thing for me to say what I believe (or think, or know, etc.), but what I in fact believe (or think, or know) is knowable only by the ascriptions of others. This is a tough, challenging theory – and to a large extent, correct.