For if it were not a cause and then became one, the coming-to-be of its causal action would itself require a cause, and that cause—or the full actuality of that cause—would in turn need another cause. This would entail an infinite regress at the very root of causality and in the completion of being a cause, and an infinite regress in the foundational act of causation is just as impossible as a regress in the very cause itself.

Thus we say to them:
Grant that such a regress is indeed impossible; yet you yourselves allow an infinite regress in the individual acts of causation and in their effects. If that regress is possible, the argument by which you seek to establish the eternity of the universe—or the eternity of any part of it—collapses, and that is precisely what we intend to show.

As for the claim that the species or kind of causation is eternal, this gains you nothing. It does not impugn what the Messengers brought; rather, it confirms it. The prophetic message is that everything other than God is created, brought into existence after non-existence, because the existence from pre-eternity of a fully sufficient cause in relation to anything whatsoever is impossible. Everything apart from God is inseparably linked to origination: its existence without the contingent events that accompany it is impossible, and the production of events by a fully sufficient cause whose effect necessarily follows it is likewise impossible. Nor can one event succeed another when both proceed from such a cause, for a cause is not fully sufficient with respect to a given thing unless that thing immediately ensues from it.

None of the successive events follows directly upon the eternal Cause, for that would entail that some of those successive events are themselves eternal—and this is impossible, for whatever coexists with the Eternal is itself eternal, without a bounded beginning that would allow something else to come after it; only what is eternal can accompany the Eternal.

Therefore, since everything other than the Lord can exist only in conjunction with a contingent event, and since no events—whether determinate or successive—can issue from an eternal, fully sufficient cause, nothing in the created order can exist except alongside