Deontic reasoning: defeasibility and norm violation

1. In [7] two approaches to formalising defeasible deontic reasoning are compared. The first is developing a special non monotonic logic for deontic statements. By way of a case study of Horty's recent work it is argued that this approach has some inherent limitations, not occurring in the second approach, combining an already existing non monotonic logic with a deontic logic. As an example of this approach the language of Reiter's default logic is extended to include modal expressions and embedded in an argument-based system previously developed.

2. In [8] standard deontic logic is extended with means to express contrary-to-duty obligations. It is shown that the system avoids some notorious paradoxes of deontic logic. Moreover, it is argued that, although use has been made of techniques that resemble techniques from non monotonic logics, contrary-to-duty reasoning is not itself non monotonic reasoning.

3. A diagnostic framework for deontic reasoning called DIODE is proposed in [13,15], based on Reiter's theory of diagnosis from first principles. The framework gives a notion of contextual obligations. A notion of absolute obligations is proposed which goes beyond the diagnostic framework and is based on Gärdenfors style retraction.
4. The notion of overridden obligations is incorporated in the DIODE framework in [14,16,17]. A multi preference semantics is proposed to model the two preferential mechanisms, ideality and normality.
5. The interference between normality and ideality in defeasible deontic logics is analysed in its generality in [18,19].
6. The representation of legal knowledge in deontic logic, in particular in DIODE and DEFDIODE, is criticised in [13,14,16]. It has been argued that contrary-to-duty obligations are fundamental for understanding the deep structure of legal knowledge, in particular for representing aggravating circumstances.