Prior’s turn from determinism to indeterminism

Indeterminism assumes a central place in Arthur Norman Prior’s invention and development of modern tense logic. Before this indeterminism, Prior was for a number of years a devout determinist. But Prior’s turn from determinism to indeterminism, so important for his mature work, has never been explained properly. This article presents the enigma of Prior’s turn from determinism to indeterminism. We know much about his early determinism and recently have learned more about how he became a determinist, but, apart from the fact that he in the course of the years between 1949 and 1953 became an indeterminist, we know very little about how or why he became an indeterminist and what exactly he then understood by free will and indeterminism. He never explained this himself at any length, but scattered remarks in various texts provide some aid in approaching this issue. We here take a look at some important texts in which Prior writes about the free will to help us a step further toward solving this riddle.