Objection 1. It would seem that the act of the sensitive appetite is not commanded. For the Apostle says (Romans 7:15): "For I do not that good which I will": and a gloss explains this by saying that man lusts, although he wills not to lust. But to lust is an act of the sensitive appetite. Therefore the act of the sensitive appetite is not subject to our command.
Objection 2. Further, corporeal matter obeys God alone, to the effect of formal transmutation, as was shown in the I, 65, 4; I, 91, 02; I, 110, 2. But the act of the sensitive appetite is accompanied by a formal transmutation of the body, consisting in heat or cold. Therefore the act of the sensitive appetite is not subject to man's command.
Objection 3. Further, the proper motive principle of the sensitive appetite is something apprehended by sense or imagination. But it is not always in our power to apprehend something by sense or imagination. Therefore the act of the sensitive appetite is not subject to our command.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xvi.] says: "That which obeys reason is twofold, the concupiscible and the irascible," which belong to the sensitive appetite. Therefore the act of the sensitive appetite is subject to the command of reason.
I answer that, An act is subject to our command, in so far as it is in our power, as stated above (Article 5). Consequently in order to understand in what manner the act of the sensitive appetite is subject to the command of reason, we must consider in what manner it is in our power. Now it must be observed that the sensitive appetite differs from the intellective appetite, which is called the will, in the fact that the sensitive appetite is a power of a corporeal organ, whereas the will is not. Again, every act of a power that uses a corporeal organ, depends not only on a power of the soul, but also on the disposition of that corporeal organ: thus the act of vision depends on the power of sight, and on the condition of the eye, which condition is a help or a hindrance to that act. Consequently the act of the sensitive appetite depends not only on the appetitive power, but also on the disposition of the body.
Now whatever part the power of the soul takes in the act, follows apprehension. And the apprehension of the imagination, being a particular apprehension, is regulated by the apprehension of reason, which is universal; just as a particular active power is regulated by a universal active power. Consequently in this respect the act of the sensitive appetite is subject to the command of reason. On the other hand, condition or disposition of the body is not subject to the command of reason: and consequently in this respect, the movement of the sensitive appetite is hindered from being wholly subject to the command of reason.
Moreover it happens sometimes that the movement of the sensitive appetite is aroused suddenly in consequence of an apprehension of the imagination of sense. And then such movement occurs without the command of reason: although reason could have prevented it, had it foreseen. Hence the Philosopher says (Polit. i, 2) that the reason governs the irascible and concupiscible not by a "despotic supremacy," which is that of a master over his slave; but by a "politic and royal supremacy," whereby the free are governed, who are not wholly subject to command.
Reply to Objection 1. That man lusts, although he wills not to lust, is due to a disposition of the body, whereby the sensitive appetite is hindered from perfect compliance with the command of reason. Hence the Apostle adds (Romans 7:15): "I see another law in my members, fighting against the law of my mind." This may also happen through a sudden movement of concupiscence, as stated above.
Reply to Objection 2. The condition of the body stands in a twofold relation to the act of the sensitive appetite. First, as preceding it: thus a man may be disposed in one way or another, in respect of his body, to this or that passion. Secondly, as consequent to it: thus a man becomes heated through anger. Now the condition that precedes, is not subject to the command of reason: since it is due either to nature, or to some previous movement, which cannot cease at once. But the condition that is consequent, follows the command of reason: since it results from the local movement of the heart, which has various movements according to the various acts of the sensitive appetite.
Reply to Objection 3. Since the external sensible is necessary for the apprehension of the senses, it is not in our power to apprehend anything by the senses, unless the sensible be present; which presence of the sensible is not always in our power. For it is then that man can use his senses if he will so to do; unless there be some obstacle on the part of the organ. On the other hand, the apprehension of the imagination is subject to the ordering of reason, in proportion to the strength or weakness of the imaginative power. For that man is unable to imagine the things that reason considers, is either because they cannot be imagined, such as incorporeal things; or because of the weakness of the imaginative power, due to some organic indisposition.
Source: Summa Theologica (New Advent)