More on Inheritance Hierarchies with Exceptions: Default Theories and Inferential Distance

In Artificial Intelligence, well-understood reasoning systems and tractable reasoning systems have often seemed mutually exclusive. This has been exemplified by nonmonotonic reasoning formalisms and inheritance-with-exceptions reasoners. These have epitomized the two extremes: the former not even semidecidable, the latter completely ad hoc. We previously presented a formal mechanism for specifying inheritance systems? and minimal criteria for acceptable inheritance reasoning. This left open the problem of realizing an acceptable reasoner. Since then. Touretzky has developed a reasoner that appears to meet our criteria. We show that his reasoner is formally adequate, and explore some of the implications of this result vis-ii-vis the study of nonmonotonic reasoning .