A crucial issue in logically formalizing common-sense reasoning is then how to represent precedence among multiple, potentially contradictory sources of information. We observe that bases for such precedence include reliability and authority. We observe furthermore that precedence relationships may themselves be inferrable and even defeasible. We show how, for the first time, to represent advice-taking, with such precedence among sources, as reasoning in a non-monotonic logical formalism. We employ a generalized variant of circumscription that we have recently developed: Defeasible Axiomatized Policy (DAP) circumscription. (See attached paper detailing DAP circumscription.) Precedence among sources is represented explicitly via the mechanism of prioritization in DAP circumscription. DAP circumscription is the first non-monotonic logical formalism to express defeasible prioritization-type precedence.
Our approach to representation of precedence among sources has a number of advantages over other non-monotonic logical formalisms. It treats precedence more explicitly than all previous approaches except perhaps Lifschitz', who was the first to address the representation of precedence among multiple sources of default information. Compared to Lifschitz', our approach has several advantages. A fundamental advantage is that ours is capable of expressing defeasible prioritization; Lifschitz' is not. Another advantage is that our approach is more adequate and more natural representationally: in particular when, as is common, the precedence partial order is not layered ("stratified"). Our approach also represents the concepts of defaults and priorities directly, unlike Lifschitz' which uses only the concept of varying versus not varying. In addition, our approach is better-behaved, e.g., with respect to satisfiability.
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