Nonmonotonic Reasoning by MonotonicInferences with Priority

The purpose of this paper is to argue that nonmonotonic reasoning in general can be viewed as monotonic inferences constrained by a simple notion of priority constraint. More important, these type of constrained inferences can be speciied in a knowledge representation language where a theory consists of a collection of logic programming-like rules and a priority constraint among them: that the application of one rule blocks that of the lower ranked rules. We thus present a formal system for representing common sense knowledge, and call it priority logic. As applications, we recast default reasoning by priority reasoning, and show that Horty's defeasible inheritance networks can be represented by priority logic. This latter result is a partial answer to Horty's challenge that it is impossible to relate path-based reasoning to general nonmono-tonic formalisms.