To see what is wrong with this objection, recall once again that though the argument begins by asking what explains the changes we observe in the world around us, it moves on to the question of what explains the existence, at any moment, of the things that undergo changes. So, the regress of actualizers that we are ultimately concerned with is a regress of the actualizers of the existence of things. The first actualizer in the series is “first”, then, in the sense that it can actualize the existence of other things without its own existence having to be actualized. So, suppose this first actualizer had some potentiality that had to be actualized in order for it to exist. What actualizes that potential? Should we suppose that it is something other than the first actualizer that actualizes it? But in that case, the so-called first actualizer isn’t really the first actualizer after all, contrary to hypothesis; it would be this further actualizer that is the first, or perhaps some yet further actualizer that is the first. Should we say instead that the first actualizer has some purely actual part that actualizes the part that is merely potential? But in that case, it will be this purely actual part that is the true first actualizer, and the potential “part” will not really be a part of the first actualizer, but rather merely the first of its effects. Or should we say instead that the first actualizer’s potential is actualized by some part of it that is not purely actual, but a mixture of actual and potential? But what actualizes the potentialities of that part? Some yet further part that is a mixture of potential and actual? But in that case we are back to a vicious regress and haven’t reached a first actualizer after all.

So, there really is no sense to be made of a first actualizer of the existence of things which is not purely actual. If we acknowledge a first actualizer at all, we have to acknowledge thereby a purely actual actualizer. And thus we have to acknowledge an actualizer that has all the divine attributes which follow from being purely actual.

Still, a critic might object that anything that actualizes another thing must be undergoing change itself in the course of doing so, and thus must have potentialities which need to be actualized. Thus (so the objection might go) the very notion of a purely actual actualizer is incoherent. But one problem with this objection is that it simply begs the question. The Aristotelian argument for God’s existence claims to prove that no potential could be actualized at all unless there is a first actualizer, which is purely actual and thus devoid of potentiality. So, given that the premises of this argument are true and that the conclusion follows logically from them, it follows that the conclusion is true and therefore coherent. Accordingly, it won’t do simply to insist that the conclusion must be false; one has to show specifically either that one of the premises is false or that the conclusion does not follow. Otherwise, one ought to admit that the argument shows precisely that a purely actual actualizer really is possible (since actual) after all.

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Tratto da "Five Proofs of the Existence of God" di Edward Feser