Showing papers by "Hector J. Levesque published in 1989"

A knowledge-level account of abduction

Abstract: In this paper, we consider a new definition of abduction that makes it depend on an underlying formal model of belief. In particular, different models of belief will give rise to different forms of abductive reasoning. Based on this definition, we then prove three main theorems: first, that when belief is closed under logical implication, the corresponding form of abduction is precisely what is performed by the ATMS as characterized by Reiter and de Kleer; second, that with the more limited "explicit" belief defined by Levesque, the required abduction is computationally tractable in certain cases where the ATMS is not; and finally, that something is believed in the implicit sense iff repeatedly applying a limited abduction operator eventually yields something that is believed in the explicit sense. This last result relates deduction and abduction as well as limited and unlimited reasoning all within the context of a logic of belief.

The tractability of path-based inheritance

Abstract: Touretzky [1984] proposed a formalism for nonmonotonic multiple inheritance reasoning that is sound in the presence of ambiguities and redundant links. We show that Touretzky's inheritance notion is NP-hard, and thus, provided P ≠ NP, computationally intractable. This result holds even when one only considers unambiguous, totally acyclic inheritance networks. A direct consequence of this result is that the conditioning strategy proposed by Touretzky to allow for fast parallel inference is also intractable. Therefore, it follows that nonmonotonic multiple inheritance hierarchies, although compact representations, may not allow for efficient retrieval of information as has been suggested in attempts to use such hierarchies, e.g., in NETL [Fahlman, 1979]. We also analyze the influence of various design choices made by Touretzky. We show that all versions of downward (coupled) inheritance, i.e., on-path or off-path preemption and skeptical or credulous reasoning, are intractable. However, tractability can be achieved when using upward (decoupled) inheritance. Thus, the main source of intractability in path-based inheritance formalisms is the downward (coupled) reasoning.

Proceedings of the first international conference on Principles of knowledge representation and reasoning

Abstract: Proceedings held May 1989. Topics include temporal logic, hierarchical knowledge bases, default theories, nonmonotonic and analogical reasoning, formal theories of belief revision, and metareasoning. Annotation copyright Book News, Inc. Portland, Or.

Knowledge representation and reasoning1

Abstract: Publisher Summary This chapter discusses the knowledge representation and reasoning hypothesis. According to the knowledge representation hypothesis, intelligent behavior ultimately depends on explicitly represented knowledge. No other design strategy as yet seems plausible to explain behavior, or have it depend in a flexible way on what one is told, or to isolate the assumptions that govern it, or to achieve any number of other desirable traits. However, KR by itself does not solve anything unless a system is able to reason effectively with what it has explicitly represented. Specifically, if reasoning needs to be performed automatically as a part of a larger task, it must be dependable both in terms of what it calculates and how long it takes. Thus, KR is the study of what information can be extracted, in a computationally dependable way, from what forms of represented knowledge. In other words, it investigates the area within the confines of the KR hypothesis.