Abstract

In nonmonotonic reasoning, the preferential system P is known to provide reasonable but very cautious conclusions, and in particular, preferential inference is blocked by the presence of “irrelevant” properties. When using Lehmann's rational closure, the inference machinery, which is then more productive, may still remain too cautious. These two types of inference can be represented using a possibility theory-based semantics. To overcome the cautiousness of system P, we first progressively augment preferential inference with two extensions which are in between system P and rational closure. Then, in order to overcome some remaining limitations of rational closure the second half of this paper focuses more particularly on the use of (contextual) independence assumptions of the form: the fact that δ is true (or is false) does not affect the validity of the rule “normally if α then β”. The modelling of such independence assumptions is discussed in the possibilistic framework. Algorithms are provided to jointly handle default rules and independence relations, and to check their multual consistency.

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