Abstract
Hypothesis theory for nonmonotonic reasoning expresses notions of hypotheses and of known information. In this paper, we define these notions in the framework of a new modal system with two modalities, one for exprressing known information and the other for expressing possible hypotheses. A complete characterization of the new logic is given in terms of Kripke semantics. Moreover, our logic allows to characterize completely default logic, including a necessary and sufficient criterium for the existence and the non-existence of extensions. We also present a notion of nonmonotonic inference which is cumulative.
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