To create is to cause existence, and all things are totally dependent upon a Creator for the very fact that they are. The Creator does not take nothing and make something out of nothing. Rather, any thing left entirely to itself, wholly separated from the cause of its existence, would be absolutely nothing. Creation is not some distant event; it is the complete causing of the existence of everything that is. Creation, thus, as Aquinas shows, is a subject for metaphysics and theology; it is not a subject for the natural sciences. Although Scripture reveals that God is Creator, for Aquinas, the fundamental understanding of creation is accessible to reason alone, in the discipline of metaphysics; it does not necessarily require faith. Aquinas thought that by starting from the recognition of the distinction between what things are, their essences, and that they are, their existence, one could reason conclusively to an absolutely first cause which causes the existence of everything that is. Aquinas shows that there are two related senses of creation, one philosophical, the other theological. The philosophical sense discloses the metaphysical dependence of everything on God as cause. The theological sense of creation, although much richer, nevertheless incorporates all that philosophy teaches and adds as well that the universe is temporally finite.
Aquinas saw no contradiction in the notion of an eternal created universe. He thought that it was a matter of biblical revelation that the world is not eternal. He also thought that reason alone could not conclude whether the world had a temporal beginning. But even if the universe were not to have had a temporal beginning, it still would depend upon God for its very being, its existence. The root sense of creation does not concern temporal origination; rather it affirms metaphysical dependence. For Aquinas, there is no conflict between the doctrine of creation and any physical theory. Theories in the natural sciences account for change. Whether the changes described are cosmological or biological, unending or finite, they remain processes. Creation accounts for the existence of things, not for changes in things. An evolving universe, just like Aristotle’s eternal universe, is still a created universe. No explanation of evolutionary change, no matter how radically random or contingent it claims to be, challenges the metaphysical account of creation, that is, of the dependence of the existence of all things upon God as cause. When some thinkers deny creation on the basis of theories of evolution, or reject evolution in defense of creation, they misunderstand creation or evolution, or both. — from “Creation, Evolution, and Thomas Aquinas [1]”