One of the world's most important political philosophers, Jon Elster is a leading thinker on reason and rationality and their roles in politics and public life. In this short book, he crystallizes and advances his work, bridging the gap between philosophers who use the idea of reason to assess human behavior from a normative point of view and social scientists who use the idea of rationality to explain behavior. In place of these approaches, Elster proposes a unified conceptual framework for the study of behavior. Drawing on classical moralists as well as modern scholarship, and using a wealth of historical and contemporary illustrations, this book marks a new development in Elster's thinking while at the same time providing a brief, elegant, and accessible introduction to his work.Jon Elster holds the Chaire de Rationalité et Sciences Sociales at the Collège de France. Among his recent books are Explaining Social Behavior and Closing the Books: Transitional Justice in Historical Perspective.