Defeasible and Pointwise Prioritization: Preliminary Report

We suggest why, and show how, to represent defeasible reasoning about prioritization-type precedence. We define Defeasible Axiomatized Policy (DAP) circumscription: it is the first formalism to express defeasible prioritization. DAP circumscription can represent one or more (generally, a finite reflective tower) of meta-levels of such reasoning, without resorting to a more powerful logical language. We argue for the usefulness, and anlyze the expressive significance, of this representational generalization. We show that it can often be achieved with only a modest increase in the mathematical complexity of inference: DAP circumscription often reduces to a series of prioritized predicate circumscriptions, for which inference procedures are currently available. DAP circumscription also offers an improved approach to pointwise prioritization and circumscription, even in the basic, monotonic case of reasoning about prioritization. We observe that unsatisfiability and representational awkwardness trouble the previous approach, due to Lifschitz [1988]. DAP circumscription overcomes these difficulties.

By: Benjamin N. Grosof

Published in: RC20125 in 1995


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