Default Reasoning

Abstract

This chapter discusses basic concepts related to default reasoning. The topic of default reasoning affords some instructive insights into the nature of the relationship between theoretical and practical reasoning. A default in logic is a fall-back position in point of conclusion-drawing—one for which it is possible to appropriately take resort when things go wrong. Orthodox inferential reasoning proceeds via logically valid inference processes which, as such, do—and must—lead to true conclusions when the premises are true. By contrast, default reasoning, which involves an information-gap between premises and conclusion, is such that premises that are true will lead to plausible (though possibly false) conclusions. The inference processes of default logic are such that the truth of the premises does not assure that of the conclusion but will at most establish that conclusion as plausible. Such inferences are ampliative: the conclusion can go beyond what the premises provide. The chapter discusses induction as default reasoning. Concepts related to default reasoning as nonmonotonic are also explained.