Catalogue of Artificial Intelligence Techniques


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Non-monotonic Reasoning

Keywords: PLANNER, closed world assumption, non-monotonic logics

Categories: Inference and Reasoning , Knowledge Representation

Author(s): Luis Jenkins , Dave Plummer

Non-monotonicity is a common feature of ordinary reasoning. For instance, if we are told that Tweety is a bird we assume Tweety can fly, but withdraw this when we are told that Tweety is a penguin. Non-monotonic reasoning can be modelled using non-monotonic logics. Such logics differ from normal deductive logics in the effects of adding new axioms. In any type of logic, new axioms may give rise to new theorems, so that the set of theorems grows with the set of axioms. In non-monotonic logics, however, the set of theorems may lose as well as gain members when new axioms are added. Default Reasoning is a way of overcoming the problem of insufficient information; the system is told that unless it has information to the contrary, certain defaults are assumed to be true. Examples of systems utilising default reasoning are PLANNER and the Negation as Failure of Prolog. In the case of Prolog, any proposition which cannot be shown to be true from the statements in database is assumed to be false: this is the so-called closed world assumption. See also Truth Maintenance System, Default Logic, and Circumscription.



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