William of Ockham's semantic theory was founded on the idea that thought takes place in a language not unlike the languages in which spoken and written communication occur. This mental language was held to have several features in common with everyday languages. For example, mental language has simple terms, not unlike words, out of which complex expressions can be constructed. As with words, each of these terms has some meaning, or signification; in fact Ockham held that the signification of everyday words derives precisely from the signification of mental terms. Furthermore, the meaning of a mental expression depends directly on the meaning of its constituent terms, as is the case with expressions in more familiar languages.

As one might expect, there are many important differences between mental language and everyday languages. For example, mental languages signify their objects naturally rather than conventionally. At a more concrete level, Ockham suggested that numerous features of spoken or written language – participles and pronouns, for example – might not exist in mental language.