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.