An eternal universe seemed incompatible with a universe created ex nihilo, and so some medieval Christians thought that Greek science, especially in the person of Aristotle, ought to be banned, since it contradicted the truths of revelation. Aquinas, believing that the truths of science and the truths of faith could not contradict one another-God being the author of all truth-went to work to reconcile Aristotelian science and Christian revelation.
The key to Aquinas’ analysis is the distinction he draws between creation and change . The natural sciences, whether Aristotelian or those of our own day, have as their subject the world of changing things: from sub-atomic particles to acorns to galaxies. Whenever there is a change there must be something that changes. The Greeks are right: from nothing, nothing comes; that is, if the verb “to come” means a change. All change requires an underlying material reality.
Creation, on the other hand, is the radical causing of the whole existence of whatever exists. To cause completely something to exist is not to produce a change in something, is not to work on or with some already existing material. If, in producing something new, an agent were to use something already existing, the agent would not by itself be the complete cause of the new thing. But such a complete causing is precisely what creation is. To create is to give existence, and all things are totally dependent upon God for the very fact that they are. God does not take nothing and make something out of “it.” Rather, anything left entirely to itself, separated from the cause of its existence, would be absolutely nothing. Creation is not some distant event; it is the continuing, complete causing of the existence of everything that is. Creation, thus, is a subject for metaphysics and theology, not for the natural sciences.