Objection 1. It would seem that the irascible and concupiscible appetites do not obey reason. For irascible and concupiscible are parts of sensuality. But sensuality does not obey reason, wherefore it is signified by the serpent, as Augustine says (De Trin. xii, 12,13). Therefore the irascible and concupiscible appetites do not obey reason.
Objection 2. Further, what obeys a certain thing does not resist it. But the irascible and concupiscible appetites resist reason: according to the Apostle (Romans 7:23): "I see another law in my members fighting against the law of my mind." Therefore the irascible and concupiscible appetites do not obey reason.
Objection 3. Further, as the appetitive power is inferior to the rational part of the soul, so also is the sensitive power. But the sensitive part of the soul does not obey reason: for we neither hear nor see just when we wish. Therefore, in like manner, neither do the powers of the sensitive appetite, the irascible and concupscible, obey reason.
On the contrary, Damascene says (De Fide Orth. ii, 12) that "the part of the soul which is obedient and amenable to reason is divided into concupiscence and anger."
I answer that, In two ways the irascible and concupiscible powers obey the higher part, in which are the intellect or reason, and the will; first, as to reason, secondly as to the will. They obey the reason in their own acts, because in other animals the sensitive appetite is naturally moved by the estimative power; for instance, a sheep, esteeming the wolf as an enemy, is afraid. In man the estimative power, as we have said above (Question 78, Article 4), is replaced by the cogitative power, which is called by some 'the particular reason,' because it compares individual intentions. Wherefore in man the sensitive appetite is naturally moved by this particular reason. But this same particular reason is naturally guided and moved according to the universal reason: wherefore in syllogistic matters particular conclusions are drawn from universal propositions. Therefore it is clear that the universal reason directs the sensitive appetite, which is divided into concupiscible and irascible; and this appetite obeys it. But because to draw particular conclusions from universal principles is not the work of the intellect, as such, but of the reason: hence it is that the irascible and concupiscible are said to obey the reason rather than to obey the intellect. Anyone can experience this in himself: for by applying certain universal considerations, anger or fear or the like may be modified or excited.
To the will also is the sensitive appetite subject in execution, which is accomplished by the motive power. For in other animals movement follows at once the concupiscible and irascible appetites: for instance, the sheep, fearing the wolf, flees at once, because it has no superior counteracting appetite.
On the contrary, man is not moved at once, according to the irascible and concupiscible appetites: but he awaits the command of the will, which is the superior appetite. For wherever there is order among a number of motive powers, the second only moves by virtue of the first: wherefore the lower appetite is not sufficient to cause movement, unless the higher appetite consents. And this is what the Philosopher says (De Anima iii, 11), that "the higher appetite moves the lower appetite, as the higher sphere moves the lower." In this way, therefore, the irascible and concupiscible are subject to reason.
Reply to Objection 1. Sensuality is signified by the serpent, in what is proper to it as a sensitive power. But the irascible and concupiscible powers denominate the sensitive appetite rather on the part of the act, to which they are led by the reason, as we have said.
Reply to Objection 2. As the Philosopher says (Polit. i, 2): "We observe in an animal a despotic and a politic principle: for the soul dominates the body by a despotic power; but the intellect dominates the appetite by a politic and royal power." For a power is called despotic whereby a man rules his slaves, who have not the right to resist in any way the orders of the one that commands them, since they have nothing of their own. But that power is called politic and royal by which a man rules over free subjects, who, though subject to the government of the ruler, have nevertheless something of their own, by reason of which they can resist the orders of him who commands. And so, the soul is said to rule the body by a despotic power, because the members of the body cannot in any way resist the sway of the soul, but at the soul's command both hand and foot, and whatever member is naturally moved by voluntary movement, are moved at once. But the intellect or reason is said to rule the irascible and concupiscible by a politic power: because the sensitive appetite has something of its own, by virtue whereof it can resist the commands of reason. For the sensitive appetite is naturally moved, not only by the estimative power in other animals, and in man by the cogitative power which the universal reason guides, but also by the imagination and sense. Whence it is that we experience that the irascible and concupiscible powers do resist reason, inasmuch as we sense or imagine something pleasant, which reason forbids, or unpleasant, which reason commands. And so from the fact that the irascible and concupiscible resist reason in something, we must not conclude that they do not obey.
Reply to Objection 3. The exterior senses require for action exterior sensible things, whereby they are affected, and the presence of which is not ruled by reason. But the interior powers, both appetitive and apprehensive, do not require exterior things. Therefore they are subject to the command of reason, which can not only incite or modify the affections of the appetitive power, but can also form the phantasms of the imagination.
Source: Summa Theologica (New Advent)