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 Matthew 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 (Question 78, Article 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.
Source: Summa Theologica (New Advent)