I came across an article on the net by Douglas Erlandson (Ph.D. in philosophy) rejecting the notion of Thomism The Resurrection of Thomism). One of his criticisms talks about the cosmological argument–which forms the core of Aquinas’ Five Proofs. In particular, he claims that the principle “ex nihil[o], nihil fit” (nothing comes from nothing) is question begging because it has to lead to the conclusion that a first un-caused cause exits, and only a theist would accept this principle. An atheist, because he believes there is no first un-caused cause, will deny the truth of the principle, and so be unmoved by Aquinas’ proofs.
I would respond to Dr. Erlandson’s critique, first of all, by pointing out that the principle “ex nihilo, nihil fit” is not the linchpin to all of the Five Ways, only of the first three, and that Aquinas has other proofs for the existence of God than the five ways. So, even if the cosmological proof were inconclusive, one may still be able to prove the existence of God on Thomistic grounds without it.
Next, I would point out that the truth of the principle does not depend on a question-begging argument for God’s existence in the way that the atheist’s denial of the principle depends on already accepting an atheist conclusion. That is, “nothing comes from nothing” is supposed to be intuitively obvious and obviuosly true, quite apart from any question of whether there is a God. While the principle may “guarantee” the conclusion that a first un-caused cause exists, its truth does not depend on this conclusion being true.
Erlandson, quite rightly, points out that for an atheist to persist in his or her atheism in the light of, say, Aquinas’ Third Way, he or she has to deny some premise of the argument, and the “ex nihilo” principle is an option. But, Aquinas would say, and I think quite reasonably, that it is just absurd to suppose that something can come from nothing. So if one wants to be an atheist, one has to accept some absurd premise. And if an atheist counters that it is not absurd that something can come from nothing, the burden of proof (for the reasonableness of this principle) is on him or her. The atheist cannot, however, appeal to the very argument (about the origin of the universe) where the reasonableness of this principle is in question to establish the reasonableness of this principle. That is, an atheist cannot say that something must come from nothing since the universe obviously has, since the origin of the universe is just what is in question. Aquinas believes, and I with him, that nothing does ever come to be without a cause. So if the atheist thinks that something does come to be without a cause, he or she has to provide some basis for this belief (other than the first contingent cause of the universe).
If one wants to critique (some) of Aquinas’ proofs for the existence of God, one cannot do so by denying “ex nihilo, nihil fit” without absurdity.