Motivation addresses a central problem in psychology: Why does an animal's behavior fluctuate in the face of an unaltered environment? In a sense this is the opposite of the question from which work on motivation began, and for which Claude Bernard invented the concept of the fixity of the internal milieu: How does an animal maintain constancy in the face of a fluctuating environment? Dealing with motivation has become extremely complex as new experiments, phenomena, and theories have extended the concept. This book embodies some of the ways in which work on motivation is currently...