The Stanford Encyclopedia of Philosophy defines a "belief" as "the attitude we have, roughly, whenever we take something to be the case or regard it as true. ... Most contemporary philosophers characterize belief as a 'propositional attitude,' [where] propositions are generally taken to be whatever it is that sentences express. For example ... 'snow is white.'" Beliefs, then, (a) are expressed in language, (b) refer to some specific contents such as "snow," and (c) express some truth about those contents. The truth need not be an empirical statement about the world -- propositions such as "x is the square root of x-squared" are also beliefs under this definition even when there is no empirical referent for "x." Beliefs can be about a single thing, or about the relationships between things, in which case they might or might not be expressed as formal rules: e.g., "every bird has wings." Language, representation...