Deontic modals

Modality, as it is usually understood in contemporary philosophy, has to do with necessities and possibilities. Deontic modality is a kind of modality which has to do with what is necessary or possible according to various rules, such as the norms of morality, the principles of practical rationality or the laws of some country. Deontic modality can be contrasted with alethic modality and epistemic modality. The former has to do with what propositions are necessarily or possibly true given various metaphysical, logical or nomological laws. The latter has to do with what propositions are necessarily or possibly true given various bodies of evidence or information.

Words commonly thought to express deontic modalities include the auxiliary verbs ‘must’, ‘have to’, ‘may’, ‘can’, ‘should’ and ‘ought to ’, but also the adjectives ‘obligatory’, ‘permissible’ and ‘impermissible’. The meanings of such deontic modals are studied by semanticists and philosophers of language interested in developing general theories of the content of modal language and concepts. The logical relations between deontic modal concepts are studied by deontic logicians interested in developing a formal language suitable for representing these relations in a logical system. The distinctive nature of deontic modals, their role in normative thought and their relations to other normative and evaluative concepts, such as reason, value, virtue and rationality, are studied by ethical theorists, especially metaethicists. All of these projects are deeply interconnected.