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Abstract

In this paper we propose an exception logic - formalizing reasoning about exceptions. We use this logic to defend two claims. First, we argue that default logic - formalizing reasoning about default assumptions - is an extension of exception logic. A deconstruction argument shows that reasoning about exceptions is one of the first principles of reasoning about default assumptions. Second, we argue that two phases have to be distinguished in reasoning about exceptions, and therefore also in reasoning about default assumptions. We identify two causes of the distinction between two phases, the disjunction rule OR and right weakening RW. This sheds some new light on these `standard' (according to the Kraus-Lehmann-Magidor paradigm) properties of default inference.