Prior’s Turn to Medieval Logic

Abstract

The peculiar aspect of medieval logic, that the truth-value of propositions changes with time, gradually disappeared as Europe exited the Renaissance. In modern logic, it was assumed by W.V.O. Quine that one cannot appreciate modern symbolic logic if one does not take it to be tenseless. A.N. Prior’s invention of tense-logic challenged Quine’s view and can be seen as a turn to medieval logic. However, Prior’s discussion of the philosophical problems related to quantified tense-logic led him to reject essential aspects of medieval logic. This invites an evaluation of Prior’s formalisation of tense-logic as, in part, an argument in favour of the medieval view of propositions. This article argues that Prior’s turn to medieval logic is hampered by his unwillingness to accept essential medieval assumptions regarding facts about objects that do not exist. Furthermore, it is argued that presentists should learn an important lesson from Prior’s struggle with accepting the implications of quantified tense-logic and reject theories that purport to be presentism as unorthodox if they also affirm Quine’s view on ontic commitment. In the widest sense: philosophers who, like Prior, turn to the medieval view of propositions must accept a worldview with facts about individuals that, in principle, do not supervene (present tense) on being, for they do not yet exist.