Ordering default theories and nonmonotonic logic programs

First-order theories are ordered under logical entailment based on the amount of information derived from theories. In default logic, on the other hand, a theory contains default information as well as definite information. To order default theories, distinguishing different sorts of information is necessary to assess the information content of a default theory. For this purpose, we first introduce a multi-valued interpretation of default theories using a ten-valued bilattice. It distinguishes between definite and credulous/skeptical default information derived from a theory, and is used for ordering default theories based on their information contents. We then apply the technique to order nonmonotonic logic programs under the answer set semantics. The results of this paper provide a method for comparing default theories or nonmonotonic logic programs in a manner different from the conventional extension/model-based viewpoint. Moreover, they have important application to induction from nonmonotonic theories.