A conviction to the truth of a proposition. Beliefs can be acquired through perception, contemplation or communication. In the psychological sense, belief is a representational mental state that takes the form of a propositional attitude.

Knowledge is often defined as justified true belief, in that the belief must be considered to correspond to reality and must be derived from valid evidence and arguments. However, this definition has been challenged by the Gettier problem which suggests that justified true belief does not provide a complete picture of knowledge.