  For many years Prior had been a Barthian Calvinist, and, as Prior put it, the theologian Karl Barth ‘attacked freewill in the name of religion’ (Prior n.d.b: 1). In his mature work, however, Prior was on the side of freewill and indeterminism. He developed profound technical analyses of the idea that the future is open, a branching tree of possibilities. The mature Prior said: