Why do human beings move? In this lucid portrayal of human behavior, Fred Dretske provides an original account of the way reasons function in the causal explanation of behavior. Biological science investigates what makes our bodies move in the way they do. Psychology is interested in why persons--agents with reasons--move in the way they do. Dretske attempts to reconcile these different points of view by showing how reasons operate in a world of causes. He reveals in detail how the character of our inner states--what we believe, desire, and intend--determines what we do.