ALTERNATIVE APPROACHES TO DEFAULT LOGIC

Reiter's default logic has proven to be an enduring and versatile appr oach to nonmonotonic reasoning. Subsequent work in default logic has c oncentrated in two major areas. First, modifications have been develop ed to extend and augment the approach. Second, there has been ongoing interest in semantic foundations for default logic. In this paper, a n umber of variants of default logic are developed to address differing intuitions arising from the original and subsequent formulations. Firs t, we modify the manner in which consistency is used in the definition of a default extension. The idea is that a global rather than local n otion of consistency is employed in the formation of a default extensi on. Second, we argue that in some situations the requirement of provin g the antecedent of a default is too strong. A second variant of defau lt logic is developed where this requirement is dropped; subsequently these approaches are combined, leading to a final variant. These varia nts then lead to default systems which conform to alternative intuitio ns regarding default reasoning. For all of these approaches, a fixed-p oint and a pseudo-iterative definition are given; as well a semantic c haracterisation of these approaches is provided. In the combined appro ach we argue also that one can now reason about a set of defaults and can determine, for example, if a particular default in a set is redund ant. We show the relation of this work to that of Lukaszewicz and Brew ka, and to the Theorist system.