Nonmonotonic logic is intended to apply specifically to situations where the initial information is incomplete. Using nonmonotonic reasoning procedures we shall be able to jump to conclusions, but withdraw them later when we gain additional information. A number of nonmonotonic logics have been introduced and widely discussed. Nonmonotonic logics tend to be introduced proof theoretically, and little attention is paid to their semantic characteristics or their computational tractability. We address both of these issues by presenting a nonmonotonic logic for the Herbrand subset of first-order predicate logic. This nonmonotonic logic is shown to be both sound and complete. Theories formulated in this logic can be executed in logic programming fashion.
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