A short reflection on Thomas Aquinas second proof for the existence of God.
The principle of sufficient causality tells us that the greater does not come from the lesser. One cannot give what one does not have. It is to this basic principle that all of the five proofs of Aquinas seem to fall back upon.
For me the case is made most strongly in the second of Aquinas’ proofs, that from efficient causality:
“There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.”
The procedure of this proof is much like that of the first proof. For in the first proof, it was also the denial of an infinite regress of accidental causes as sufficient that was expressed. In both proofs, Aquinas need not demonstrate that there is in fact no infinite regress in accidental causes. This is important, for Aquinas likewise never thought that it was contrary to reason to posit an infinite past; an eternal universe (revelation tells us there was a beginning, but reason cannot say one way or the other; here Thomas disagrees with, for example, his friend St. Bonaventure, who would say that the world’s beginning can be known by reason alone)
But in point of fact, it is not the infinite past and series of causes that is as important as the essence of the cause. If we look at the first argument, from motion, we could in some way imagine a clock with a large number of cogs. We can perhaps explain the rotation of a certain cog by the force applied to it by another cog, but we cannot explain the rotation of all the cogs merely by the large number of cogs. In fact, we could in some way imagine an infinite amount of cogs, yet this would not suddenly explain the motion of the entire series. Rather, something outside the series must explain it. Likewise, we cannot move a train, long or short, by adding one more boxcar. We must add an engine, something completely unique.
The difference in the second argument (from causality) from the first (motion) is that it seems to tell us a little more about God. It is closer to letting us contemplate the fact that God is pure act, with no potency. The act/potency distinction may well be the most important in all of Aquinas’ philosophical arsenal. Knowing God as pure act, and therefore necessary and self-subsisting being, is probably the primary key to all of Thomas Aquinas’ reflections in the Summa from Q.3 through Q.26, known as the Treatise on the One God. Almost everything we can know of God by reason alone is a by-product, so to speak, of our unpacking what it means to be pure act, self-subsisting being.