This proof is not, as some imagine, predicated on the impossibility of an infinite series of effects. Those who advance it do not deny the endless succession of effects; they deny only what every rational person rejects: an infinite regress of efficient causes and an infinite regress in that which is the very root of causation. Confusion often arises between the completion of a cause for a particular effect and the completion of causation as such. Their argument rests on the impossibility of a complete cause coming to be from pre-eternity, for nothing comes into existence unless its full, operative cause has itself come into existence. Hence, were the entire class of temporal events to have had a beginning, the occurrence of that first event would require a complete cause that itself had begun to exist, and that complete cause would call for yet another, and so on without end.

To clarify the reply, we say that infinite regress (tasalsul) is of two kinds.

1. An infinite regress of efficient causes. All rational people agree that this is impossible; its impossibility is evident to sound reason, and a clear conception of it suffices to see its falsity.

2. An infinite regress of effects, where an agent perpetually produces one effect after another. On this point there are three well-known views: (a) that it is impossible both eternally and everlastingly, (b) that it is permissible both eternally and everlastingly, and (c) that it is impossible from pre-eternity yet permissible without end in the future.

An infinite series of effects may entail an infinite series in the completion of particular acts of causation, but not in the roots of causation itself. For if the emergence of a complete cause is conditioned on the occurrence of a prior effect, then the completion of causation for the second affair depends on the appearance of the first effect—just as with all moving bodies, such as the sun, the moon, and the like, where the second segment of a movement relies on the issuing of the first.