lntroducing generalized specificity in logic programming

Most formalisms for representing common-sense knowledge allow incomplete and potentially inconsistent information. When strong negation is also allowed, contradictory conclusions can arise. Therefore, a criterion for deciding between them is needed. Several extensions of logic programming consider priorities over program (default) rules. However, these priorities must be supplied by the programmer in a more or less arbitrary manner, establishing explicitly relations between rules. The aim of this paper is to investigate beyond explicit comparison between rules, looking for an inherent and autonomous comparison criterion, based on specificity as defined in [22, 25]. In contrast to other approaches, we consider not only defeasible, but also strict knowledge. Our criterion for comparing arguments, namely specificity, is context-sensitive. This means that preference among defeasible rules is determined dynamically during the dialectical analysis. We show how such a specificity criterion can be defined in terms of two different approaches: activation sets and derivation trees. This allows us to get a more syntactic criterion that can be implemented in a computationally attractive way. The resulting definitions may be applied in a generic rule-based formalism. We present a theorem which links both characterizations, showing their equivalence. Finally we discuss other frameworks for defeasible reasoning in which preference handling is considered explicitly, contrasting them with our approach.

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