Thomas Aquinas, Compendium of Theology

We might imagine that, although God can produce a new effect by His eternal and immutable will, some sort of motion would have to precede the newly produced effect. For we observe that the will does not delay doing what it wishes to do, unless because of some motive that is operative now but will cease later, or because of some motive that is inoperative now but is expected to become operative in the future. In summer a man has the will to clothe himself with a warm garment, which, however, he does not wish to put on at present, but in the future; for now the weather is warm, although it will cease to be warm with the advent of a cold wave later in the year.

Accordingly, if God wished from eternity to produce some effect, but did not produce it from eternity, it seems either that something was expected to happen in the future that had not yet occurred, or else that some obstacle had to be removed that was then present. Neither of these alternatives can take place without motion. Thus it seems that a subsequent effect cannot be produced by a preceding will unless some motion previously occurs. And so, if God's will relative to the production of things was eternal, and nevertheless things were not produced from eternity, their production must have been preceded by motion, and consequently by mobile objects. And if the latter were produced by God, but not from eternity, yet other motions and mobile objects must have preceded, and so on, in infinite recession.

The solution to this objection readily comes to mind if we but attend to the difference between a universal and a particular agent. A particular agent has an activity that conforms to a norm and measure prescribed by the universal agent. This is clear even in civil government. The legislator enacts a law which is to serve as a norm and measure. Any particular judge must base his decisions on this law. Again, time is the measure of actions which occur in time. A particular agent is endowed with activity regulated by time, so that he acts for some definite reason now, and not before. But the universal agent, God, instituted this measure, which is time, and He did so in accord with His will. Hence time also is to be numbered among the things produced by God. Therefore, just as the quantity and measure of each object are such as God wishes to assign to it, so the quantity of time is such as God wished to mete out; that is, time and the things existing in time began just when God wished them to begin.

The objection we are dealing with argues from the standpoint of an agent that presupposes time and acts in time, but did not institute time. Hence the question, why God's eternal will produces an effect now and not earlier, presupposes that time exists; for "now" and "earlier" are segments of time. With regard to the universal production of things, among which time is also to be counted, we should not ask: "Why now and not earlier?" Rather we should ask: "Why did God wish this much time to intervene?" And this depends on the divine will, which is perfectly free to assign this or any other quantity to time. The same may be noted with respect to the dimensional quantity of the world. No one asks why God located the material world in such and such a place rather than higher up or lower down or in some other position; for there is no place outside the world. The fact that God portioned out so much quantity to the world that no part of it would be beyond the place occupied in some other locality, depends on the divine will. However, although there was no time prior to the world and no place outside the world, we speak as if there were. Thus we say that before the world existed there was nothing except God, and that there is no body lying outside the world. But in thus speaking of "before" and "outside," we have in mind nothing but time and place as they exist in our imagination.