Permissibility and Justifying Reasons in the Formal Objection to Moral Error Theory

Abstract Moral error theory has recently faced formal objections. Since it denies the existence of moral properties that are defined in contradistinction to each other (e.g. permissibility and impermissibility), it appears to fall into incoherence. While some have rejected the duality of interdefinable deontic modals in response to this objection, I defend moral error theory while upholding this duality. Drawing on the distinction between ‘weak’ and ‘strong’ permissibility in relation to requiring and justifying reasons, my analysis shows that any plausible formulation of deontic duals must define a strong deontic modal in contradistinction to a weaker dual. This allows moral error theory to deny propositions that ascribe a strong deontic status to action, while affirming those that ascribe only a weak status.