RICHARD A. POSNER, a judge of the United States Court of Appeals for the Seventh Circuit and a senior lecturer at the University of Chicago Law School, is a man who believes that there is always a good reason for the things people do and that by applying the principles of rationality one can find it. He is also a very clever man who responds to the world in a manner that is nothing if not ingenious. In his view, human behavior is primarily the result of choices, conscious or unconscious, in pursuit of interests, and if we come to understand the conditions that both give rise to and constrain such choices, we will, like so many philosopher- kings, be able to devise a truly reasonable system of laws and regulations.