We therefore reply: His very essence is what brings about each single one of those utterances and acts; yet its influence upon the second is conditioned by the non-existence of the first.

Were someone to insist that, when the second contingent event comes to be, an additional positive occurrence must also arise, this would necessitate an infinite regress in the completion of causation—a regress within causes themselves—which is impossible. For if that event could not occur unless, at the very moment of its appearance, another contingent appeared, and that in turn depended on still another, there would result an endless chain of completed causes and effects. Such a regress is plainly impossible by sound reason and by the consensus of rational minds, just as it is impossible for every originated thing to have a cause, and that cause another, without end.

Hence all contingent events must proceed from an Eternal Being, and the coming-to-be of the second is tied to the disappearance of the first. The only new circumstance on the side of the agent is precisely the absence of the first, while the Eternal Cause produces one event after another; thus the emergence of the second is conditioned upon the non-existence of the first.

It may be objected: What you have just stated must be considered with respect to the First Agent, necessarily existent in Himself, if He brings things into being successively and is not the complete cause of any eternal entity; for His essence entails the kind of causation, not a specific, eternal act of causation.

Those who advocate the world’s pre-eternity have confused the genus of causation with its particular instances. Seeing that the Divine Essence necessarily implies being a cause—and deeming it impossible for that implication to begin in time—they failed to distinguish between the general capacity to cause and a specific act of causing, and so imagined that this required the co-eternity of the celestial spheres or other concrete entities within the universe.