A Paper Abstract by Benjamin Grosof

Prioritizing Multiple, Contradictory Sources in Common-Sense Learning By Being Told; or, Advice-Taker Meets Bureaucracy (Jan. 11 1993)

by Benjamin N. Grosof

Abstract: An important common-sense capability is to learn by "taking advice", or "being told", from a variety of sources of information: messages from various other agents, reading various texts, etc.. Such learned or "assimilated" knowledge provides an important basis for one's actions. Yet a basic feature of life is that one cannot believe everything that one is told. Not only may advice be incorrect; e.g., it may be contradicted by direct experience. Worse, different sources of advice information may contradict each other, or even themselves. How is an advice-taking agent to maintain a consistent, yet usefully actionable, set of beliefs, then? Here, we address this problem, which has not received much previous attention in the AI literature. We illustrate, with an actual example, how it arises in the domain of bureaucracy. A common human strategy is to treat advice as working belief, which can be overridden by other advice and direct experience. This suggests the first element of our approach: to represent advice as default-status knowledge.

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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