  Aristotle believed that most reasoning, including reasoning about what to do and about sublunary natural phenomena, dealt with things that hold “always or for the most part.” But Aristotelian logic deals only with patterns of inference that hold without exception. We find at the very beginning of logic a discrepancy between the scope of logical theory and common sense reasoning. Nonmonotonic logic is the first sustained attempt within logical theory to remedy this discrepancy. As such, it represents a potential for a sweeping expansion of the scope of logic, as well as a significant body of technical results.