I am grateful to the John Simon Guggenheim Memorial Foundation and to the National Science Foundation (grant no. SES-8701534) fo r their support of this project. Many colleagues unselfishly offered their advice on earlier drafts of this book; to one and all I offer in return my sincere appreciation, but I must especially thank Mike Friedman, Al Janis, David Malament, Tim Maudlin, John Norton, Robert Rynasiewicz, and Paul Teller. Thanks are also due to Michael Wright, whose generous support of the �-Club made it possible to travel to England, where I received both encouragement and helpful criticism from Harvey Brown, Jeremy Butterfield, Michael Redhead, Simon Saunders, and other members of the Oxbridge mob. Other colleagues could have, but didn't, offer help; here I would like especially to mention Larry Sklar. However, his superb selec tions in Chinese restaurants more than make up for this lapse. Section 5.1 relies on "Remarks on Relational Theories of Motion,"