It is proposed that the cognitive system imposes patterns on the world according to a simplicity principle: Choose the pattern that provides the briefest representation of the available information. The simplicity principle is normatively justified-patterns that support simple representations provide good explanations and predictions on the basis of which the agent can make decisions and actions. Moreover, the simplicity principle appears to be consistent with empirical data from many psychological domains, including perception, similarity, learning, memory, and reasoning. Thus, the simplicity principle promises to serve as the starting point for the rational analysis of a wide range of cognitive processes, in Anderson's (1990, 1991a) sense. The simplicity principle also provides a framework for integrating a wide range of existing psychological proposals.