DefDiode: a diagnostic model for defeasible deontic logic

There is a fundamental difference between a conditional obligation being violated by a fact, and a conditional obligation being overridden by another conditional obligation. In this paper we propose a multi preference semantics for a defeasible deontic logic that is based on this fundamental difference. The semantics contains one preference relation for ideality, which can be used to formalize deontic `paradoxes' like the Chisholm and Forrester `Paradoxes', and another preference relation for normality, which can be used to formalize exceptions. The interference of the two preference orderings generates new questions about preferential semantics.