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.