St. Thomas Aquinas' Third Way Modalized

Aquinas' Third Way Modalized

ABSTRACT: The Third Way is the most interesting and insightful of Aquinas' five arguments for the existence of God, even though it is invalid and has some false premises. With the help of a somewhat weak modal logic, however, the Third Way can be transformed into a argument which is certainly valid and plausibly sound. Much of what Aquinas asserted in the Third Way is possibly true even if it is not actually true. Instead of assuming, for example, that things which are contingent fail to exist at some time, we need only assume that contingent things possibly fail to exist at some time. Likewise, we can replace the assumption that if all things fail to exist at some time then there is a time when nothing exists, with the corresponding assumption that if all things possibly fail to exist at some time then possibly there is a time when nothing exists. These and other similar replacements suffice to produce a cogent cosmological argument.

Aquinas' Third Way is a cosmological argument for the existence of God which "is taken from possibility and necessity." It is surprising therefore that philosophers of religion have not shown much interest in applying modal logic to its analysis. (1) There are a couple of reasons. First, Aquinas does not always use the words 'possibility' and 'necessity' in the same way that they are used in modal logic. Second, cosmological arguments generally purport to build a bridge between some property of this world and a supreme being, making it unnecessary, it might be thought, to appeal to modalized features of other possible worlds.

Modal logic has of course been applied extensively to the analysis of ontological arguments. Ontological arguments purport to build a logical bridge between thought and a supreme being. Most ontological arguments proceed from the assumption that it is possible for God to exist. They then link this assumption with some rather "strong" and controversial principles of modal logic in order to prove that God must exist in all possible worlds, from which it follows that God exists in the real world. (2) It might be possible, however, to prove the existence of God with the use of a weak and noncontroversial system of modal logic if we root the proof with some plausible possibilistic principles about what might be true of the cosmos.

The Third Way is not sound per se.