Wolfgang Spohn presents the first full account of the dynamic laws of belief, by means of ranking theory. This book is his long-awaited presentation of ranking theory and its ramifications. He motivates and introduces the basic notion of a ranking function, which recognises degrees of belief and at the same time accounts for belief simpliciter. He provides a measurement theory for ranking functions, accounts for auto-epistemology in ranking-theoretic terms, and explicates the basic notion of a (deductive or non-deductive) reason. The rich philosophical applications of Spohn's theory include: a new account of lawlikeness, an account of ceteris paribus laws, a new perspective on dispositions, a rich and detailed theory of deterministic causation, an understanding of natural modalities as an objectification of epistemic modalities, an account of the experiential basis of belief—and thus a restructuring of the debate on foundationalism and coherentism (and externalism and contextualism)—and, finally, a revival of fundamental a priori principles of reason fathoming the basics of empiricism and the relation between reason and truth, and concluding in a proof of a weak principle of causality. All this is accompanied by thorough comparative discussions, on a general level as well as within each topic, and in particular with respect to probability theory.