An analysis of St. Thomas Aquinas's 'third way

St. Thomas Aquinas's "third way" of proving God's existence (in Summa theologiae 1, q. 2, a. 3) has two parts. The first establishes the existence of "some necessary being." The second moves from there to establish a divine necessary being that causes necessity in other beings. The first part has occasioned considerable controversy owing to two troublesome propositions. The first of these states that whatever is capable of not-existing does not exist at some time. The second infers that if all individual things were possibles then at some time nothing at all would have existed. The former proposition has been criticized for overlooking the possibility that something which is merely capable of not existing might nevertheless always actually exist. The latter has been charged with committing a composition fallacy.;After a critical survey of previous interpretations, our study defends these two troublesome propositions by reading the proof in light of Thomas's particular cosmology and natural philosophy. On this interpretation, all possibles are either bodies materially composed of the four elements, or the elements themselves insofar as they remain in contact with contrary elements. In defense of the first problematic proposition, we argue that for Thomas all such possible beings are necessarily subject to generation and corruption and therefore must not exist at some time. The second proposition is defended in a similar way. The inference "if all things are possibles, then at some time nothing existed," can be read as implicitly removing the causal influence of necessary beings on possible ones. We thus reconstruct this inference by appealing to Thomas's teaching that the incorruptible heavens are conserving causes of the substantial forms of lower bodies.;Concerning the proof's second part, we argue that Thomas's identification of God as a being that causes necessity in other necessary beings is fitting since such causality is tantamount to creation of these other beings ex nihilo. His argumentation for the existence of such a cause is lacking, however, insofar as it does not sufficiently establish that the "some necessary being" reached by the proof's first part actually needs a cause of its necessity.