There is a nonmonotonic logic which: (i) is suitable for default reasoning; (ii) came right from the beginning with an implementation; and (iii) is more general than other approaches which have been proposed recently. In fact this logic has been known for over 10 years now: it is Nute's defeasible logic. Despite these obvious advantages the logic is not in the mainstream of nonmonotonic reasoning, mainly because it has not been studied sufficiently as a formal system, and because its relationship to other formalisms has not been studied up to now.

This paper seeks to change the perception of defeasible logic. In particular, it presents the logic in a formal setting, derives several properties, including representational results, and gives an operational semantics. Finally the paper discusses some relationships of defeasible logic to other default reasoning approaches.