Objection 1. It would seem that the sin of consent to the act is not in the higher reason. For consent is an act of the appetitive power, as stated above (Question 15, Article 1): whereas the reason is an apprehensive power. Therefore the sin of consent to the act is not in the higher reason.
Objection 2. Further, "the higher reason is intent on contemplating and consulting the eternal law," as Augustine states (De Trin. xii, 7). ['Rationes aeternae,' cf. I, 15, A2, where as in similar passages 'ratio' has been rendered by the English 'type,' because St. Thomas was speaking of the Divine 'idea' as the archetype of the creature. Hence the type or idea is a rule of conduct, and is identified with the eternal law, (cf. 8, Objection 1; 9)]. But sometimes consent is given to an act, without consulting the eternal law: since man does not always think about Divine things, whenever he consents to an act. Therefore the sin of consent to the act is not always in the higher reason.
Objection 3. Further, just as man can regulate his external actions according to the eternal law, so can he regulate his internal pleasures or other passions. But "consent to a pleasure without deciding to fulfil it by deed, belongs to the lower reason," as Augustine states (De Trin. xii, 2). Therefore the consent to a sinful act should also be sometimes ascribed to the lower reason.
Objection 4. Further, just as the higher reason excels the lower, so does the reason excel the imagination. Now sometimes man proceeds to act through the apprehension of the power of imagination, without any deliberation of his reason, as when, without premeditation, he moves his hand, or foot. Therefore sometimes also the lower reason may consent to a sinful act, independently of the higher reason.
On the contrary, Augustine says (De Trin. xii, 12): "If the consent to the evil use of things that can be perceived by the bodily senses, so far approves of any sin, as to point, if possible, to its consummation by deed, we are to understand that the woman has offered the forbidden fruit to her husband."
I answer that, Consent implies a judgment about the thing to which consent is given. For just as the speculative reason judges and delivers its sentence about intelligible matters, so the practical reason judges and pronounces sentence on matters of action. Now we must observe that in every case brought up for judgment, the final sentence belongs to the supreme court, even as we see that in speculative matters the final sentence touching any proposition is delivered by referring it to the first principles; since, so long as there remains a yet higher principle, the question can yet be submitted to it: wherefore the judgment is still in suspense, the final sentence not being as yet pronounced. But it is evident that human acts can be regulated by the rule of human reason, which rule is derived from the created things that man knows naturally; and further still, from the rule of the Divine law, as stated above (Question 19, Article 4). Consequently, since the rule of the Divine law is the higher rule, it follows that the ultimate sentence, whereby the judgment is finally pronounced, belongs to the higher reason which is intent on the eternal types. Now when judgment has to be pronounced on several points, the final judgment deals with that which comes last; and, in human acts, the action itself comes last, and the delectation which is the inducement to the action is a preamble thereto. Therefore the consent to an action belongs properly to the higher reason, while the preliminary judgment which is about the delectation belongs to the lower reason, which delivers judgment in a lower court: although the higher reason can also judge of the delectation, since whatever is subject to the judgment of the lower court, is subject also to the judgment of the higher court, but not conversely.
Reply to Objection 1. Consent is an act of the appetitive power, not absolutely, but in consequence of an act of reason deliberating and judging, as stated above (Question 15, Article 3). Because the fact that the consent is finally given to a thing is due to the fact that the will tends to that upon which the reason has already passed its judgment. Hence consent may be ascribed both to the will and to the reason.
Reply to Objection 2. The higher reason is said to consent, from the very fact that it fails to direct the human act according to the Divine law, whether or not it advert to the eternal law. For if it thinks of God's law, it holds it in actual contempt: and if not, it neglects it by a kind of omission. Therefore the consent to a sinful act always proceeds from the higher reason: because, as Augustine says (De Trin. xii, 12), "the mind cannot effectively decide on the commission of a sin, unless by its consent, whereby it wields its sovereign power of moving the members to action, or of restraining them from action, it become the servant or slave of the evil deed."
Reply to Objection 3. The higher reason, by considering the eternal law, can direct or restrain the internal delectation, even as it can direct or restrain the external action: nevertheless, before the judgment of the higher reason is pronounced the lower reason, while deliberating the matter in reference to temporal principles, sometimes approves of this delectation: and then the consent to the delectation belongs to the lower reason. If, however, after considering the eternal law, man persists in giving the same consent, such consent will then belong to the higher reason.
Reply to Objection 4. The apprehension of the power of imagination is sudden and indeliberate: wherefore it can cause an act before the higher or lower reason has time to deliberate. But the judgment of the lower reason is deliberate, and so requires time, during which the higher reason can also deliberate; consequently, if by its deliberation it does not check the sinful act, this will deservedly by imputed to it.
Source: Summa Theologica (New Advent)