This book elaborates, modifies, and extends some of the themes of the first three chapters of Enterprise of Knowledge. I was prompted to return to these matters by reading the excellent paper of Carlos Alchourròn, Peter Gärdenfors, and David Makinson, “On the Logic of Theory Change: Partial Meet Functions for Contraction and Revision,” and the substantial literature anticipating and emerging from this important paper. In addition, Gärdenfors presented me with an excellent target of criticism. Thanks to his book Knowledge in Flux, we have available a superb introduction to the contemporary discussion of belief change from the best-known student of the subject. To a very considerable degree, my book focuses on the points of disagreement between Gärdenfors and myself. I want to emphasize at the outset, therefore, my profound admiration for the quality of his work and the work of his colleagues Alchourròn and Makinson, and my gratitude to Gärdenfors for his collegial friendship and his generous assistance to me in writing this book. He has written extensive notes on all of the chapters, and I have relied on them heavily in preparing the final version.

As time goes by, I become more indebted to my former student, current colleague, and good friend, Teddy Seidenfeld. He sent me detailed comments on the first draft that have helped me clarify many obscurities and improve the final version.

While writing Chapter 2, I found enlightenment and stimulation in speaking with Akeel Bilgrami, Arnold Koslow, and Shaughan Lavine.