Specificity and Inheritance in Default Reasoning

Abstract

When specificity considerations are incorporated in default reasoning systems, it is hard to ensure that exceptional subclasses inherit all legitimate features of their parent classes. To reconcile these two requirements, specificity and inheritance, this paper proposes the addition of a new rule, called coherence rule, to the desiderata for default inference. The coherence rule captures the intuition that formulae which are more compatible with the defaults in the database are more believable. We offer a formal definition of this extended desiderata and analyze the behavior of its associated closure relation which we ca ll coherence closure. We provide a concrete embodiment of a system satisfying the extended desiderata, by taking the coherence closure of system Z . A procedure for computing the (unique) most compact belief ranking in the coherence closure of system Z is also described.