Max Planck Institute for Computer Science
Erasmus University Rotterdam
P.O. Box 1738
3000 DR Rotterdam
The deontic logic dus is a deontic update semantics for prescriptive obligations based on the update semantics of Veltman. In dus the definition of logical validity of obligations is not based on static truth values but on dynamic action transitions. In this paper prescriptive prima facie obligations are formalized in update semantics. The logic formalizes the specificity principle, has reinstatement and does not have an irrelevance problem. Moreover, it handles the diagnostic problem by distinguishing between overridden, conflict and factual defeasibility.