Objection 1. It would seem that consent to the act does not always belong to the higher reason. For "delight follows action, and perfects it, just as beauty perfects youth" [oion tois akmaiois he hora--as youthful vigor perfects a man in his prime] (Ethic. x, 4). But consent to delight belongs to the lower reason, as Augustine says (De Trin. xii, 12). Therefore consent to the act does not belong only to the higher reason.
Objection 2. Further, an act to which we consent is said to be voluntary. But it belongs to many powers to produce voluntary acts. Therefore the higher reason is not alone in consenting to the act.
Objection 3. Further, "the higher reason is that which is intent on the contemplation and consultation of things eternal," as Augustine says (De Trin. xii, 7). But man often consents to an act not for eternal, but for temporal reasons, or even on account of some passion of the soul. Therefore consent to an act does not belong to the higher reason alone.
On the contrary, Augustine says (De Trin. xii, 12): "It is impossible for man to make up his mind to commit a sin, unless that mental faculty which has the sovereign power of urging his members to, or restraining them from, act, yield to the evil deed and become its slave."
I answer that, The final decision belongs to him who holds the highest place, and to whom it belongs to judge of the others; for as long as judgment about some matter remains to be pronounced, the final decision has not been given. Now it is evident that it belongs to the higher reason to judge of all: since it is by the reason that we judge of sensible things; and of things pertaining to human principles we judge according to Divine principles, which is the function of the higher reason. Wherefore as long as a man is uncertain whether he resists or not, according to Divine principles, no judgment of the reason can be considered in the light of a final decision. Now the final decision of what is to be done is consent to the act. Therefore consent to the act belongs to the higher reason; but in that sense in which the reason includes the will, as stated above (01, ad 1).
Reply to Objection 1. Consent to delight in the work done belongs to the higher reason, as also does consent to the work; but consent to delight in thought belongs to the lower reason, just as to the lower reason it belongs to think. Nevertheless the higher reason exercises judgment on the fact of thinking or not thinking, considered as an action; and in like manner on the delight that results. But in so far as the act of thinking is considered as ordained to a further act, it belongs to the lower reason. For that which is ordained to something else, belongs to a lower art or power than does the end to which it is ordained: hence the art which is concerned with the end is called the master or principal art.
Reply to Objection 2. Since actions are called voluntary from the fact that we consent to them, it does not follow that consent is an act of each power, but of the will which is in the reason, as stated above (01, ad 1), and from which the voluntary act is named.
Reply to Objection 3. The higher reason is said to consent not only because it always moves to act, according to the eternal reasons; but also because it fails to dissent according to those same reasons.
Source: Summa Theologica (New Advent)