This book by one of the world's foremost philosophers in the fields of epistemology and logic offers an account of suppositional reasoning relevant to practical deliberation, explanation, prediction, and hypothesis testing. Suppositions made "for the sake of the argument" sometimes conflict with our beliefs, and when they do, some beliefs are rejected and others retained. Thanks to such hypothetical belief contravention, adding content to a supposition can undermine conclusions reached without it. Subversion can also arise because suppositional reasoning is ampliative. These two types of nonmonotonicity are the focus of this book. A detailed comparison of nonmonotonicity appropriate to both belief-contravening and ampliative suppositional reasoning reveals important differences that have been overlooked. In arguing that the distinction between belief contravening and inductive nonmonotonicity plays a far greater role in deliberation and decision than it is given credit for, this major study will be required reading for all philosophers and logicians concerned with conditionals, decision theory, and inductive inference. It will also interest those in artificial intelligence who work on expert systems, default reasoning, and nonmonotonic reasoning.