The philosophical logic of Obligations and Permissions. Used in agent systems and rule based systems.
Taken from ``Deontic Logic In Computer Science'' (Preface) ISBN 0471937436
"Deontic logic is a branch of ModalLogic? to reason about normative versus non-normative behaviour by means of modal operators such as ought, permitted and forbidden.
Originating from philosophy where it is used to formalize (reasoning about) notions in ehtics and philosophy of laws, it has become apparent recently that a more profane application of deontic logic can also be found in certain areas of computer science and artificial intelligence. For example, it provides a very adequate tool to describe integrity constraints of information systems, particularly those dealing with obligations and prohibitions. However, in other applications deontic logic is also useful, as diverse as office systems, fault-tolerant systems, security policies and natural language processing. Futhermore, from the roots mentioned above its use in legal expert systems seems obvious as well.
Many of the above applications can be arranged under the common denominator of normative systems. By this we mean systems in the behaviour of which norms play a role and which need normative concepts in order to be described or specified. Until recently in specifications of systems in computional environments the distinction between normative behaviour (as it should be) and actual behaviour (as it is) has been disregarded: mostly it is not possible to specify that some system behaviour is non-normative (illegal) but nevertheless possible. Often illegal behaviour is just ruled out by the specification, although it is very important to be able to specify what should happpen if such illegal but possible behaviour occurs! Deontic logic provides a means to do just this by using special modal operators that indicate the status of behaviour: that is whether it is legal (normative) or not."
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