Logical approaches to nonomotonic reasoning have been developed within different technical settings, thus making it difficult to establish correspondences among them and to identify common underlying 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 natrual generalization of Reiter's default logic, which fully captures both closure assumptions. We then analyze the relationship between GDL and nonomonotonic modal logics, in particular Moore's autoepistemic logic and Lifschitz's logic of minimal knowledge and negation as failure, showing the exitence of a full correspondence between these modal formalisms and GDL. SUch a corresondence given us a unified reading of nonmotonic reasoning formalisms in terms of the above two assumptions; in particular, it clarifies the relationship between default and autoepistemic logic.
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