How to reason defeasibly

• Published in: Artificial intelligence Vol. 57; no. 1; pp. 1 - 42
• Main Author: Pollock, John L.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 1992; Elsevier
• Subjects: Applied sciences; Artificial intelligence; Computer science; control theory; systems; Exact sciences and technology; Problem solving, game playing; System evaluation; First order logic; Reasoning; Inference; Automatic proving; Rationalization; Computer system; Artificial intelligence
• ISSN: 0004-3702; 1872-7921
• DOI: 10.1016/0004-3702(92)90103-5

This paper describes the construction of a general-purpose defeasible reasoner that is complete for first-order logic and provably adequate for the argument-based conception of defeasible reasoning that I have developed elsewhere. Because the set of warranted conclusions for a defeasible reasoner will not generally be recursively enumerable, a defeasible reasoner based upon a rich logic like the predicate calculus cannot function like a traditional theorem prover and simply enumerate the warranted conclusions. An alternative criterion of adequacy called i.d.e.-adequacy is formulated. This criterion takes seriously the idea that defeasible reasoning may involve indefinitely many cycles of retracting and reinstating conclusions. It is shown how to construct a reasoner that, subject to certain realistic assumptions, is provably i.d.e.-adequate. The most recent version of OSCAR implements this system, and examples are given of OSCAR's operation.