For many commonsense reasoning tasks associated with action domains, only a relatively simple kind of causal knowledge is required - knowledge of the conditions under which facts are caused. This note introduces a modal nonmonotonic logic for representing causal knowledge of this kind, relates it to other nonmonotonic formalisms, and shows that a variety of causal theories of action can e expressed in it, including the recently proposed causal action theories of Lin. The new logic extends the causal theories of McCain and Turner, and provides a more adequate semantic account of it. A useful subset of the logic has a concise translation into classical propositional logic, and so can be used for automated planning and reasoning about action. A larger subset is closely related to logic programming under the answer set semantics, yielding another approach to automated reasoning.
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I am happily indebted to Norm McCain for years of conversation and collaboration on topics related to this paper. I thank participants of the Mini-Workshop on Causality (UT Austin, May 5–6, 1996) for challenging discussions. I am deeply grateful to my Ph.D. advisor, Vladimir Lifschitz, for generous guidance and encouragement. Thanks to Enrico Giunchiglia for timely advice and criticism, and to the anonymous referees for helpful suggestions. This work was partially supported by National Science Foundation Grant IRI-9306751 and University of Minnesota Grant-in-aid #17831.