The purpose of this paper is to find an interpretation on which several of Aquinas’ metaphysical commitments are consistent. One commitment is that God is his essence; another is that God is his existence (i.e. his being); and the last is that God is simple. Aquinas concludes from the first two that, in God, essence and existence are identical. The first objection in Summa Theologiae
3.4 in effect leads this conclusion in a pantheistic direction. For, it challenges Aquinas to find a difference between God’s existence and the existence of everything else. If Aquinas cannot find such a difference then, if God’s existence and essence are the same and if God’s existence is the same as the existence of everything else, then it follows that God’s essence is the same as the existence of everything else, which means that everything has God’s essence. This leads to a type of pantheism that Aquinas surely would not embrace.
In this paper I argue that there is exactly one possible way in which God’s existence is different from the existence of everything else: God’s existence is pure actuality whereas the existence of everything else is actuality but not pure actuality. This argument requires me to enumerate several of Aquinas’ metaphysical principles, some found in the Summa Theologiae, others in On Being and Essence, which I do in the first half of the paper. In the second half I show why the difference between these two types of existence can only be in terms of actuality.