Many-Valued Logic

Abstract

Throughout the orthodox mainsteam of the development of logic in the West, the prevailing view was that every proposition is either true or else false (although which of these is the case may well neither be necessary as regards the matter itself nor determinable as regards our knowledge of it). This thesis, commonly called the ‘law of bivalence’ — constituting one key articulation of the ‘law of the excluded middle’ — was, however, already questioned in antiquity. In Ch. IX of his treatise On Interpretation (De interpretatione), Aristotle discussed of the truth-status of alternatives regarding ‘future-contingent’ matters, whose occurrence — like that of the sea-battle tomorrow — is not yet determinable by us, and may indeed actually be undetermined. His views on the matter are still disputed, but many commentators, both in antiquity and later, held him to assert that propositions about future contingents, like that asserting the occurrence of the sea-battle, are neither actually true nor actually false, but potentially either, thus having — at least prior to the event — a third, indeterminate truth-status. The acceptance of the principle of bivalence was, in antiquity, closely bound up with the doctrine of determinism. The Epicureans, who were indeterminists, rejected the law of bivalence; the Stoics (and above all Chrysippus) who were rigid determinists, insisted upon it.¹