Aquinas knew that, even from before Christianity, philosophers had distinguished two kinds of causation with regard to the "cause of being" of the world:
(1) a causation which is “preservative of being with a beginning in time,” in the sense that there was nothing before the material world was brought into existence by the cause of being; and
(2) a causation which is “preservative of being without a beginning in time,” in the sense that there was never a time when there was nothing in existence because the world is begininglessly eternal.
The former type of causation of being is properly speaking creation. The latter type of causation means only conservation of being.
In the first type of causation there are two actvities, creation and preservation.
In the second type of causation there is only one activity, preservation.
For one thing is the activity that causes the staying in existence of things ‘which have always existed;’ and another, the activity that causes the coming into existence of things ‘which begin to exist for the first time’ out of no pre-existing materials.
The activity of holding in existence a begininglessly eternal world is not creation. And this is the type of causation that Aquinas attributes to 'his Aristotle.'
Here Aquinas explicitly declares himself at variance with 'his Aristotle,' for Aquinas embraces as his own the causation of being with a beginning in time.
A careful reading of Aquinas shows that he is not as imprecise as it may appear on the issue of how the world came to exist. Aquinas’ understanding of the notions involved when addressing this issue and the contexts in which the issue is confronted leave no room for doubt as to where Aquinas stands with respect to other philosophers and as to what he wanted to say himself about the creation of the world.