Paraconsistency and deontic logic: Formal systems for reasoning with normative conflicts.

摘要

This dissertation presents and investigates some deontic logics (logics of obligation and permission) that tolerate normative conflicts, i.e., situations in which something is both obligatory and forbidden, or both forbidden and permitted. Standard systems of deontic logic break down in the presence of normative conflicts: in them, the existence of a single normative conflict entails that everything is obligatory (as well as forbidden). Since, as I argue at length, normative conflicts are possible, standard deontic logic should be rejected.;In this dissertation, I specify and evaluate some alternatives to standard deontic logic that are conflict-tolerant by virtue of the fact that they are constructed on the basis of paraconsistent logics---logics that tolerate contradictions without "exploding" into triviality. Paraconsistent deontic logics are superior to standard deontic logic in most ways. However, such systems appear to be unsatisfactory in certain respects---for example, they invalidate the classically valid inference known as disjunctive syllogism.;As an alternative, I propose a new approach to constructing a conflict-tolerant deontic logic: a sort of compromise between standard deontic logic and paraconsistent deontic logic which I call "semi-paraconsistent" deontic logic. I highlight some interesting features of semi-paraconsistent deontic logics, the most notable of which is that they tolerate normative conflicts while preserving all of classical propositional logic. I also examine the question of whether semi-paraconsistent deontic logic is philosophically well-motivated. I conclude that, while the semi-paraconsistent approach is subject to at least one quite powerful objection, it nevertheless is the most satisfactory approach proposed to date, and suggests some promising new lines of research.