A theoretical cornerstone of cognitive therapy is the idea that the relationship between experience and cognition, the interpretation of experience, is recursive. Human beings can respond to a given experience with more than one description and/or explanation of the experience and these alternative interpretations can have different experiential consequences. A cognitive interpretation which is repeatedly or habitually produced in response to a variety of experiences is a belief. This paper uses the idea of a recursive relationship between experience and cognition to propose a heuristic model of believing as a purposeful activity by which the human organism adapts to its environment. The belief "Earth is being explored by extraterrestrials" is used as an example in describing the four stages of this model: (1) generating an interpretation of an experience; (2) justifying the interpretation; (3) enacting the interpretation; and (4) evaluating the interpretation vis-a-vis experiential consequences. Interrelationships among the stages are explained. The practice of cognitive therapy is discussed in terms of the model, and sociocultural implications of the model are considered. (NB)