Nonmonotonic logics are meant to be a formalization of nonmonotonic reasoning. However, for the most part they fail to capture in a perspicuous fashion two of the most important aspects of such reasoning: the explicit computational nature of nonmonotonic inference, and the assignment of preferences among competing inferences. We propose a method of nonmonotonic reasoning in which the notion of inference from specific bodies of evidence plays a fundamental role. The formalization is based on autoepistemic logic, but introduces additional structure, a hierarchy of evidential subtheories. The method offers a natural formalization of many different applications of nomnonotonic reasoning, including reasoning about action, speech acts, belief revision, and various situations involving competing defaults.