Objection 1. It would seem that choice is an act, not of will but of reason. For choice implies comparison, whereby one is given preference to another. But to compare is an act of reason. Therefore choice is an act of reason.
Objection 2. Further, it is for the same faculty to form a syllogism, and to draw the conclusion. But, in practical matters, it is the reason that forms syllogisms. Since therefore choice is a kind of conclusion in practical matters, as stated in Ethic. vii, 3, it seems that it is an act of reason.
Objection 3. Further, ignorance does not belong to the will but to the cognitive power. Now there is an "ignorance of choice," as is stated in Ethic. iii, 1. Therefore it seems that choice does not belong to the will but to the reason.
On the contrary, The Philosopher says (Ethic. iii, 3) that choice is "the desire of things in our power." But desire is an act of will. Therefore choice is too.
I answer that, The word choice implies something belonging to the reason or intellect, and something belonging to the will: for the Philosopher says (Ethic. vi, 2) that choice is either "intellect influenced by appetite or appetite influenced by intellect." Now whenever two things concur to make one, one of them is formal in regard to the other. Hence Gregory of Nyssa [Nemesius, De Nat. Hom. xxxiii.] says that choice "is neither desire only, nor counsel only, but a combination of the two. For just as we say that an animal is composed of soul and body, and that it is neither a mere body, nor a mere soul, but both; so is it with choice."
Now we must observe, as regards the acts of the soul, that an act belonging essentially to some power or habit, receives a form or species from a higher power or habit, according as an inferior is ordained by a superior: for if a man were to perform an act of fortitude for the love of God, that act is materially an act of fortitude, but formally, an act of charity. Now it is evident that, in a sense, reason precedes the will and ordains its act: in so far as the will tends to its object, according to the order of reason, since the apprehensive power presents the object to the appetite. Accordingly, that act whereby the will tends to something proposed to it as being good, through being ordained to the end by the reason, is materially an act of the will, but formally an act of the reason. Now in such like matters the substance of the act is as the matter in comparison to the order imposed by the higher power. Wherefore choice is substantially not an act of the reason but of the will: for choice is accomplished in a certain movement of the soul towards the good which is chosen. Consequently it is evidently an act of the appetitive power.
Reply to Objection 1. Choice implies a previous comparison; not that it consists in the comparison itself.
Reply to Objection 2. It is quite true that it is for the reason to draw the conclusion of a practical syllogism; and it is called "a decision" or "judgment," to be followed by "choice." And for this reason the conclusion seems to belong to the act of choice, as to that which results from it.
Reply to Objection 3. In speaking "of ignorance of choice," we do not mean that choice is a sort of knowledge, but that there is ignorance of what ought to be chosen.
Source: Summa Theologica (New Advent)