“Quod possibile est non esse quandoque non est”. Aquinas’ Third Way in the light of Hintikka’s Principle of Plenitude

Abstract

According to both Jaakko Hintikka and Simo Knuuttila, Aquinas’ third way to demonstrate that God exists presupposes the acceptance of the principle of plenitude, i.e., of the claim that all possibilities are realized at some time. Aquinas, however, maintained elsewhere that not all possibilities are always realized, and the coherence of his philosophical project may be called into question if one were to accept Hintikka’s and Knuuttila’s reading of the third way. In this paper, I argue that it is difficult to present the third way without invoking the principle of plenitude in Hintikka’s formulation. The corollary of this claim is that third way cannot be a demonstration within the philosophical system outlined by Aquinas, despite his claim to the contrary. Against the backdrop of this exegetical discussion, it is possible to rephrase Aquinas’ third way as a probabilistic argument that shows that God’s existence is highly likely, although not necessarily proven.