"This is the first logically precise, computationally implementable, book-length account of rational belief revision. It explains how a rational agent ought to proceed when adopting a new belief -- a difficult matter if the new belief contradicts the agent's old beliefs...This account affords a novel way to argue that there is an inviolable core of logical principles. These principles, which form the system of Core Logic, cannot be given up, on pain of not being able to carry out the reasoning involved in rationally revising beliefs. The book ends by comparing and contrasting the new account with some major representatives of earlier alternative approaches, from the fields of formal epistemology, artificial intelligence and mathematical logic." -- Book jacket.