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modality
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Aristotle
Aristotle
Aristotle Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle, c. 325 bce; in the collection of the Roman National Museum.

modality

logic
Also known as: alethic modality
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modality, in logic, the classification of logical propositions according to their asserting or denying the possibility, impossibility, contingency, or necessity of their content. Modal logic, which studies the logical features of such concepts, originated with Aristotle, was extensively studied by logicians in antiquity and the European Middle Ages, and, for the most part, was neglected after the Renaissance until revived in modern mathematical logic. The basic statement on this subject, presupposed in most contemporary discussions, is by C.I. Lewis and Cooper Harold Langford in Symbolic Logic (1932), which develops a modal system of “strict implication” for interpreting the logical force of “if . . . then.”

This article was most recently revised and updated by Brian Duignan.
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deontic logic
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deontic logic

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deontic logic, Branch of modal logic that studies the permitted, the obligatory, and the forbidden, which are characterized as deontic modalities (Greek, deontos: “of that which is binding”). It seeks to systematize the abstract, purely conceptual relations between propositions in this sphere, such as the following: If an act is obligatory, then its performance must be permitted and its omission forbidden. In given circumstances, every act is such that either it or its omission is permitted. Modal logic leaves to substantive disciplines such as ethics and law the concrete questions of what specific acts or states of affairs are to be forbidden, permitted, or the like.

This article was most recently revised and updated by Brian Duignan.
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