To believe a proposition is to hold it to be true. The philosophical problem is to understand what kind of state of a person constitutes belief. Is it, for example, a simple disposition to behaviour? Or a more complex state that resists identification with any such disposition? Is verbal skill or verbal behaviour essential to belief, in which case what is to be said about prelinguistic infants, or nonlinguistic animals? An evolutionary approach asks how the cognitive success of possessing the capacity to believe things relates to success in practice. Further topics include discovering whether belief differs from other varieties of assent, such as acceptance, discovering whether belief is an all-or-nothing matter, or to what extent degrees of belief are possible, understanding the ways in which belief is controlled by rational and irrational factors, and discovering its links with other properties, such as the possession of conceptual or linguistic skills.