This book deals with the subject of probabilistic reasoning. Professor Pollock attempts to make general philosophical sense of objective probabilities and he explores their relationship to the problem of induction. He argues that probability is fundamental not only to physical science, but to induction, epistemology, the philosophy of science and to much reasoning of interest to artificial intelligence. He attempts to show how his understanding of probability throws light on these related issues. Pollock's main claim is that the fundamental notion of probability is nomic - that is, it involves the notion of natural law, valid across possible worlds. The various epistemic and statistical conceptions of probability, he demonstrates, are derived from this nomic notion. He goes on to provide a theory of statistical induction; an account of computational principles allowing some probabilities to be derived from others; an account of acceptance rules; and a theory of direct inference.