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Abstract

In this article we introduce Contextual Deontic Logic (CDL) to analyze the relation between deontic, contextual and defeasible reasoning. The optimal state, and therefore the set of active obligations, can change radically when the violation context changes. In such cases we say that the obligations only in force in the previous violation context are defeated; contextual deontic logic is therefore a defeasible deontic logic. This is expressed by the definition OC(A|B) = O(A|B\-C): `A ought to be (done) if B is (done) in the context where C is (done)' is defined as `A ought to be (done) if B is (done) unless -C is (done).' The unless clause formalizes explicit exceptions and is analogous to the justification in Reiter's default rules. CDL is a monotonic defeasible deontic logic, because it has factual defeasibility but not overridden defeasibility.