Objection 1. It seems that some cause can be assigned to the divine will. For Augustine says (Qq. lxxxiii, 46): "Who would venture to say that God made all things irrationally?" But to a voluntary agent, what is the reason of operating, is the cause of willing. Therefore the will of God has some cause.
Objection 2. Further, in things made by one who wills to make them, and whose will is influenced by no cause, there can be no cause assigned except by the will of him who wills. But the will of God is the cause of all things, as has been already shown (4). If, then, there is no cause of His will, we cannot seek in any natural things any cause, except the divine will alone. Thus all science would be in vain, since science seeks to assign causes to effects. This seems inadmissible, and therefore we must assign some cause to the divine will.
Objection 3. Further, what is done by the willer, on account of no cause, depends simply on his will. If, therefore, the will of God has no cause, it follows that all things made depend simply on His will, and have no other cause. But this also is not admissible.
On the contrary, Augustine says (Qq. lxxxiii, 28): "Every efficient cause is greater than the thing effected." But nothing is greater than the will of God. We must not then seek for a cause of it.
I answer that, In no wise has the will of God a cause. In proof of which we must consider that, since the will follows from the intellect, there is cause of the will in the person who wills, in the same way as there is a cause of the understanding, in the person that understands. The case with the understanding is this: that if the premiss and its conclusion are understood separately from each other, the understanding the premiss is the cause that the conclusion is known. If the understanding perceive the conclusion in the premiss itself, apprehending both the one and the other at the same glance, in this case the knowing of the conclusion would not be caused by understanding the premisses, since a thing cannot be its own cause; and yet, it would be true that the thinker would understand the premisses to be the cause of the conclusion. It is the same with the will, with respect to which the end stands in the same relation to the means to the end, as do the premisses to the conclusion with regard to the understanding.
Hence, if anyone in one act wills an end, and in another act the means to that end, his willing the end will be the cause of his willing the means. This cannot be the case if in one act he wills both end and means; for a thing cannot be its own cause. Yet it will be true to say that he wills to order to the end the means to the end. Now as God by one act understands all things in His essence, so by one act He wills all things in His goodness. Hence, as in God to understand the cause is not the cause of His understanding the effect, for He understands the effect in the cause, so, in Him, to will an end is not the cause of His willing the means, yet He wills the ordering of the means to the end. Therefore, He wills this to be as means to that; but does not will this on account of that.
Reply to Objection 1. The will of God is reasonable, not because anything is to God a cause of willing, but in so far as He wills one thing to be on account of another.
Reply to Objection 2. Since God wills effects to proceed from definite causes, for the preservation of order in the universe, it is not unreasonable to seek for causes secondary to the divine will. It would, however, be unreasonable to do so, if such were considered as primary, and not as dependent on the will of God. In this sense Augustine says (De Trin. iii, 2): "Philosophers in their vanity have thought fit to attribute contingent effects to other causes, being utterly unable to perceive the cause that is shown above all others, the will of God."
Reply to Objection 3. Since God wills effects to come from causes, all effects that presuppose some other effect do not depend solely on the will of God, but on something else besides: but the first effect depends on the divine will alone. Thus, for example, we may say that God willed man to have hands to serve his intellect by their work, and intellect, that he might be man; and willed him to be man that he might enjoy Him, or for the completion of the universe. But this cannot be reduced to other created secondary ends. Hence such things depend on the simple will of God; but the others on the order of other causes.
Source: Summa Theologica (New Advent)