A Nonmonotonic Extension to Horn-Clause Logic

Standard first-order logic, and therefore, any description of the world that relies on it, ordinarily requires the absence of contradiction. It has been repeatedly pointed out that in describing the real world it is often very difficult to avoid making contradictory statements (e.g., when realizing that an assumption had proven wrong, we might have to assert the denial of what we had assumed to hold true). The example that has become a classic in the AI literature is the problem that Birds fly, Penguins are birds, but Penguins don’t fly.