The Nature of Nonmonotonic Reasoning
 Charles G. Morgan
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Conclusions reached using common sense reasoning from a set of premises are often subsequently revised when additional premises are added. Because we do not always accept previous conclusions in light of subsequent information, common sense reasoning is said to be nonmonotonic. But in the standard formal systems usually studied by logicians, if a conclusion follows from a set of premises, that same conclusion still follows no matter how the premise set is augmented; that is, the consequence relations of standard logics are monotonic. Much recent research in AI has been devoted to the attempt to develop nonmonotonic logics. After some motivational material, we give four formal proofs that there can be no nonmonotonic consequence relation that is characterized by universal constraints on rational belief structures. In other words, a nonmonotonic consequence relation that corresponds to universal principles of rational belief is impossible. We show that the nonmonotonicity of common sense reasoning is a function of the way we use logic, not a function of the logic we use. We give several examples of how nonmonotonic reasoning systems may be based on monotonic logics.
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 Title
 The Nature of Nonmonotonic Reasoning
 Journal

Minds and Machines
Volume 10, Issue 3 , pp 321360
 Cover Date
 20000801
 DOI
 10.1023/A:1026560718525
 Print ISSN
 09246495
 Online ISSN
 15728641
 Publisher
 Kluwer Academic Publishers
 Additional Links
 Topics
 Keywords

 logic
 nonclassical logic
 nonmonotonic logic
 Authors

 Charles G. Morgan ^{(1)} ^{(2)}
 Author Affiliations

 1. Department of Philosophy, University of Victoria, Victoria, B.C, V8W 3P4, Canada
 2. Varney Bay Institute for Advanced Study, P.O. Box 45, Coal Harbour, B.C. VON 1KO, Canada