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Studia Logica

, Volume 57, Issue 1, pp 91–115 | Cite as

Contrary-to-duty obligations

  • Henry Prakken
  • Marek Sergot
Article

Abstract

We investigate under what conditions contrary-to-duty (CTD) structures lacking temporal and action elements can be given a coherent reading. We argue, contrary to some recent proposals, that CTD is not an instance of defeasible reasoning, and that methods of nonmonotonic logics are inadequate since they are unable to distinguish between defeasibility and violation of primary obligations. We propose a semantic framework based on the idea that primary and CTD obligations are obligations of different kinds: a CTD obligation pertains to, or pre-supposes, a certain context in which a primary obligation is already violated. This framework is presented initially as an extension of Standard Deontic Logic (SDL), a normal modal logic of type KD, and is illustrated by application to a series of examples. The concluding section is concerned with some resemblances between CTD and defeasible reasoning. We show first that the SDL-based framework contains a flaw and must be adjusted. A discussion of possible adjustments, including an alternative treatment in terms of a preference-based semantics, reveals difficulties that are reminiscent of problems in defeasible reasoning and intensional accounts of defeasible conditionals.

Key words

deontic logic deontic conditionals contrary-to-duty obligations reparational obligations formalisation of norms 

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Copyright information

© Kluwer Academic Publishers 1996

Authors and Affiliations

  • Henry Prakken
    • 1
  • Marek Sergot
    • 2
  1. 1.Computer/Law InstituteFree University AmsterdamHV Amsterdam
  2. 2.Department of ComputingImperial College of Science Technology and MedicineLondon

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