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A tutorial on default logics

Published:01 December 1999Publication History
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Abstract

Default logic is one of the most prominent approaches to nonmonotonic reasoning, and allows one to make plausible conjectures when faced with incomplete information about the problem at hand. Default rules prevail in many application domains such as medical and legal reasoning.

Several variants have been developed over the past year, either to overcome some perceived deficiencies of the original presentation, or to realize somewhat different intuitions. This paper provides a tutorial-style introduction to some important approaches of Default Logic. The presentation is based on operational models for these approaches, thus making them easily accessible to a broader audience, and more easily usable in practical applications.

References

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  1. A tutorial on default logics

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    Doris C. Appleby

    Antoniou continues the generally excellent series of surveys published by the ACM for over 30 years. Each is intended for upper-level undergraduates or beginning graduate students, and describes a particular area of computer science. This tutorial focuses on the basic concepts of default reasoning, which is the logic that applies to a system that has only incomplete information at hand, and where plausible conjectures called defaults are added to the premises being assumed. If new information later becomes available, the default and any conclusions depending on its use may be retracted. Such a system is nonmonotonic. That is, if a statement s follows from a set of premises M , and M?M ? , then s does not necessarily follow from M ? . Only a general familiarity with the predicate calculus is assumed. The <__?__Pub Caret>monograph is based on work beginning with that of <__?__Pub Fmt nolinebreak>Reiter<__?__Pub Fmt /nolinebreak> (1980), continuing through Lukaszewicz (1988–90), and ending with the Rational Default Logic of Mikitiuk and <__?__Pub Fmt nolinebreak>Trusczynski <__?__Pub Fmt /nolinebreak>(1995). A 1996 paper of Engelfriet and Treur on the semantics of default logics is included in the excellent reference list, but statement meanings are not discussed in this tutorial. A simple example neatly explains the difference between a default logic and classical logic (p.<__?__Pub Fmt interword-space>339). Default logic uses football: ¬ snow takesPlace , whereas classical has football ? ¬ snow ? takesPlace . In the first rule, ¬ snow is used as a default, whereas in the second, it is a fact. Classical reasoning would prohibit any football games from being scheduled unless it were “proven” that there would be no snow, while using a default allows games to be listed assuming that there will be no snow. Although this example was well chosen, others are old chestnuts, such as the bird tweety series. I was disappointed by the lack of motivation for using default logics in real-world situations, such as databases, law, and medicine, as the author claims. The tweety example is not sufficient. The author states that the purpose of this tutorial is to contribute to “the understanding of the basic concepts” (p.<__?__Pub Fmt interword-space>358), and he refers readers to specific references discussing implementations. He notes that default reasoning has proven difficult for students at the University of Toronto to understand. I believe that the inclusion of better examples would have helped.

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    • Published in

      cover image ACM Computing Surveys
      ACM Computing Surveys  Volume 31, Issue 4
      Dec. 1999
      145 pages
      ISSN:0360-0300
      EISSN:1557-7341
      DOI:10.1145/344588
      Issue’s Table of Contents

      Copyright © 1999 ACM

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      Publication History

      • Published: 1 December 1999
      Published in csur Volume 31, Issue 4

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