A Note on the Rational Closure of Knowledge Bases with Both Positive and Negative Knowledge

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The notion of the rational closure of a positive knowledge base K of conditional assertions θi |∼ φi (standing for if θithen normally φi) was first introduced by Lehmann (1989) and developed by Lehmann and Magidor (1992). Following those authors we would also argue that the rational closure is, in a strong sense, the minimal information, or simplest, rational consequence relation satisfying K. In practice, however, one might expect a knowledge base to consist not just of positive conditional assertions, θi |∼ φi, but also negative conditional assertions, θi $$ φi (standing for not{if θithen normally φi}). Restricting ourselves to a finite language we show that the rational closure still exists for satisfiable knowledge bases containing both positive and negative conditional assertions and has similar properties to those exhibited in Lehmann and Magidor (1992). In particular an algorithm in Lehmann and Magidor (1992) which constructs the rational closure can be adapted to this case and yields, in turn, completeness theorems for the conditional assertions entailed by such a mixed knowledge base.

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