The intended point is that nothing in the world precedes the events whose occurrence is certain; rather, each thing exists either together with them or after them. Hence, if every constituent of the world necessarily entails events, that alone suffices to make the case. For were any such constituent eternal, it could only be so by virtue of a particular eternal act that brought it—and the events that accompany it—into existence. Yet its existence without those events is impossible, and an eternal act that produces the events is likewise impossible. Therefore, an eternal act that produces what entails them is impossible, and consequently the eternality of what entails them is impossible—that is precisely what was to be proven.

If you look at the concomitance from both sides you will say: producing the events by means of an eternal act is impossible, and producing them without their simultaneous counterpart is also impossible. Thus, whenever they are brought into being, they must be produced along with that counterpart—and that could happen only through an eternal act, which is impossible.

The matter is obvious whichever way you turn it. Suppose the entailing entity were produced by one eternal act and the events by another eternal act; the result would be that both acts are eternal, for neither can be separated from the other. Just as the two effects are inseparable, so too are the two acts. If the eternity of either of the two concomitants is impossible, the eternity of the other is likewise impossible; for were its eternity possible, we would be forced to accept either the joint existence of both—which is impossible—or the existence of one without the other—which is equally impossible.

It has therefore become clear that—even while allowing, indeed even while affirming, the possibility of an infinite series of events with no beginning—the eternality of the world, or of any part of it, remains impossible. The difference is now evident between the everlasting existence of the Necessary Being in Himself—eternal, self-subsistent, and in need of nothing—and the continuous succession of His act or of what He brings about. The Former—glorified be He—is eternal in His very essence, necessarily existent, and independent, whereas His act consists of entities that come to be one after another.