AI*IA 99: Advances in Artificial Intelligence

Logical approaches to nonmonotonic reasoning have been developed within di erent technical settings thus making it di cult to establish correspondences among them and to identify common under lying principles In this paper we argue that the most well known non monotonic reasoning formalisms are actually characterized by two closure assumptions a minimal knowledge assumption and an autoepistemic as sumption We justify this thesis by introducing generalized default logic GDL obtained through a simple and natural generalization of Reit er s default logic which fully captures both closure assumptions We then analyze the relationship between GDL and nonmonotonic modal logics in particular Moore s autoepistemic logic and Lifschitz s logic of minimal knowledge and negation as failure showing the existence of a full correspondence between these modal formalisms and GDL Such a correspondence gives us a uni ed reading of nonmonotonic reasoning for malisms in terms of the above two assumptions in particular it clari es the relationship between default and autoepistemic logic