Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
Next we have to consider the power of sensuality, concerning which there are three points of inquiry:
(1) Whether sensuality is only an appetitive power?
(2) Whether it is divided into irascible and concupiscible as distinct powers?
(3) Whether the irascible and concupiscible powers obey reason?
Objection 1: It would seem that sensuality is not only appetitive, but also cognitive. For Augustine says (De Trin. xii, 12) that "the sensual movement of the soul which is directed to the bodily senses is common to us and beasts." But the bodily senses belong to the apprehensive powers. Therefore sensuality is a cognitive power.
Objection 2: Further, things which come under one division seem to be of one genus. But Augustine (De Trin. xii, 12) divides sensuality against the higher and lower reason, which belong to knowledge. Therefore sensuality also is apprehensive.
Objection 3: Further, in man's temptations sensuality stands in the place of the "serpent." But in the temptation of our first parents, the serpent presented himself as one giving information and proposing sin, which belong to the cognitive power. Therefore sensuality is a cognitive power.
On the contrary, Sensuality is defined as "the appetite of things belonging to the body."
I answer that, The name sensuality seems to be taken from the sensual movement, of which Augustine speaks (De Trin. xii, 12, 13), just as the name of a power is taken from its act; for instance, sight from seeing. Now the sensual movement is an appetite following sensitive apprehension. For the act of the apprehensive power is not so properly called a movement as the act of the appetite: since the operation of the apprehensive power is completed in the very fact that the thing apprehended is in the one that apprehends: while the operation of the appetitive power is completed in the fact that he who desires is borne towards the thing desirable. Therefore the operation of the apprehensive power is likened to rest: whereas the operation of the appetitive power is rather likened to movement. Wherefore by sensual movement we understand the operation of the appetitive power: so that sensuality is the name of the sensitive appetite.
Reply to Objection 1: By saying that the sensual movement of the soul is directed to the bodily senses, Augustine does not give us to understand that the bodily senses are included in sensuality, but rather that the movement of sensuality is a certain inclination to the bodily senses, since we desire things which are apprehended through the bodily senses. And thus the bodily senses appertain to sensuality as a preamble.
Reply to Objection 2: Sensuality is divided against higher and lower reason, as having in common with them the act of movement: for the apprehensive power, to which belong the higher and lower reason, is a motive power; as is appetite, to which appertains sensuality.
Reply to Objection 3: The serpent not only showed and proposed sin, but also incited to the commission of sin. And in this, sensuality is signified by the serpent.
Objection 1: It would seem that the sensitive appetite is not divided into the irascible and concupiscible as distinct powers. For the same power of the soul regards both sides of a contrariety, as sight regards both black and white, according to the Philosopher (De Anima ii, 11). But suitable and harmful are contraries. Since, then, the concupiscible power regards what is suitable, while the irascible is concerned with what is harmful, it seems that irascible and concupiscible are the same power in the soul.
Objection 2: Further, the sensitive appetite regards only what is suitable according to the senses. But such is the object of the concupiscible power. Therefore there is no sensitive appetite differing from the concupiscible.
Objection 3: Further, hatred is in the irascible part: for Jerome says on Mat. 13:33: "We ought to have the hatred of vice in the irascible power." But hatred is contrary to love, and is in the concupiscible part. Therefore the concupiscible and irascible are the same powers.
On the contrary, Gregory of Nyssa (Nemesius, De Natura Hominis) and Damascene (De Fide Orth. ii, 12) assign two parts to the sensitive appetite, the irascible and the concupiscible.
I answer that, The sensitive appetite is one generic power, and is called sensuality; but it is divided into two powers, which are species of the sensitive appetite---the irascible and the concupiscible. In order to make this clear, we must observe that in natural corruptible things there is needed an inclination not only to the acquisition of what is suitable and to the avoiding of what is harmful, but also to resistance against corruptive and contrary agencies which are a hindrance to the acquisition of what is suitable, and are productive of harm. For example, fire has a natural inclination, not only to rise from a lower position, which is unsuitable to it, towards a higher position which is suitable, but also to resist whatever destroys or hinders its action. Therefore, since the sensitive appetite is an inclination following sensitive apprehension, as natural appetite is an inclination following the natural form, there must needs be in the sensitive part two appetitive powers---one through which the soul is simply inclined to seek what is suitable, according to the senses, and to fly from what is hurtful, and this is called the concupiscible: and another, whereby an animal resists these attacks that hinder what is suitable, and inflict harm, and this is called the irascible. Whence we say that its object is something arduous, because its tendency is to overcome and rise above obstacles. Now these two are not to be reduced to one principle: for sometimes the soul busies itself with unpleasant things, against the inclination of the concupiscible appetite, in order that, following the impulse of the irascible appetite, it may fight against obstacles. Wherefore also the passions of the irascible appetite counteract the passions of the concupiscible appetite: since the concupiscence, on being aroused, diminishes anger; and anger being roused, diminishes concupiscence in many cases. This is clear also from the fact that the irascible is, as it were, the champion and defender of the concupiscible when it rises up against what hinders the acquisition of the suitable things which the concupiscible desires, or against what inflicts harm, from which the concupiscible flies. And for this reason all the passions of the irascible appetite rise from the passions of the concupiscible appetite and terminate in them; for instance, anger rises from sadness, and having wrought vengeance, terminates in joy. For this reason also the quarrels of animals are about things concupiscible---namely, food and sex, as the Philosopher says [*De Animal. Histor. viii.].
Reply to Objection 1: The concupiscible power regards both what is suitable and what is unsuitable. But the object of the irascible power is to resist the onslaught of the unsuitable.
Reply to Objection 2: As in the apprehensive powers of the sensitive part there is an estimative power, which perceives those things which do not impress the senses, as we have said above (Q[78], A[2]); so also in the sensitive appetite there is a certain appetitive power which regards something as suitable, not because it pleases the senses, but because it is useful to the animal for self-defense: and this is the irascible power.
Reply to Objection 3: Hatred belongs simply to the concupiscible appetite: but by reason of the strife which arises from hatred, it may belong to the irascible appetite.
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 (Rom. 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 (Q[78], A[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.