We have argued that there is a first efficient cause, and that there is only one such cause. It follows that nothing can escape the causality of this cause, not even things that we would usually think of as caused by chance or by free will.
It is clear from experience that human beings make choices. However, whatever we might say about our freedom in these choices, we cannot avoid admitting that the first cause not only causes us to exist, but also to make the choices that we make. St. Thomas describes this situation:
The divine will imposes necessity on some things willed but not on all. The reason of this some have chosen to assign to intermediate causes, holding that what God produces by necessary causes is necessary; and what He produces by contingent causes contingent. This does not seem to be a sufficient explanation, for two reasons.
First, because the effect of a first cause is contingent on account of the secondary cause, from the fact that the effect of the first cause is hindered by deficiency in the second cause, as the sun’s power is hindered by a defect in the plant. But no defect of a secondary cause can hinder God’s will from producing its effect.
Secondly, because if the distinction between the contingent and the necessary is to be referred only to secondary causes, this must be independent of the divine intention and will; which is inadmissible. It is better therefore to say that this happens on account of the efficacy of the divine will. For when a cause is efficacious to act, the effect follows upon the cause, not only as to the thing done, but also as to its manner of being done or of being. Thus from defect of active power in the seed it may happen that a child is born unlike its father in accidental points, that belong to its manner of being. Since then the divine will is perfectly efficacious, it follows not only that things are done, which God wills to be done, but also that they are done in the way that He wills. Now God wills some things to be done necessarily, some contingently, to the right ordering of things, for the building up of the universe. Therefore to some effects He has attached necessary causes, that cannot fail; but to others defectible and contingent causes, from which arise contingent effects. Hence it is not because the proximate causes are contingent that the effects willed by God happen contingently, but because God prepared contingent causes for them, it being His will that they should happen contingently.
We could compare this situation to the situation of an author with respect to the imaginary world that he creates. Thus I can write the following story:
Peter Smith was out for a walk with Michael Jones. They came to a fork in the path. Peter, having the freedom to go to the left or to the right, freely chose to go to the right. Michael, however, was under the mental influence of aliens and had no choice in the matter. They compelled him to go to the left, and so he and Peter parted ways.
Note that in this story, Peter is free and Michael is not. And this happens because I decided that it was going to happen that way. Since I am responsible for whole of their imaginary existence, I am also responsible for the fact that one of them has an imaginary free existence, and one does not. In the same way, the first cause is responsible not only for the things that exist, but for everything which is true about the things that exist, including whatever kind of freedom or contingency that they might possess.