Objection 1. It would seem that we ought to distinguish irascible and concupiscible parts in the superior appetite, which is the will. For the concupiscible power is so called from "concupiscere" [to desire], and the irascible part from "irasci" [to be angry]. But there is a concupiscence which cannot belong to the sensitive appetite, but only to the intellectual, which is the will; as the concupiscence of wisdom, of which it is said (Wisdom 6:21): "The concupiscence of wisdom bringeth to the eternal kingdom." There is also a certain anger which cannot belong to the sensitive appetite, but only to the intellectual; as when our anger is directed against vice. Wherefore Jerome commenting on Matthew 13:33 warns us "to have the hatred of vice in the irascible part." Therefore we should distinguish irascible and concupiscible parts of the intellectual soul as well as in the sensitive.
Objection 2. Further, as is commonly said, charity is in the concupiscible, and hope in the irascible part. But they cannot be in the sensitive appetite, because their objects are not sensible, but intellectual. Therefore we must assign an irascible and concupiscible power to the intellectual part.
Objection 3. Further, it is said (De Spiritu et Anima) that "the soul has these powers"--namely, the irascible, concupiscible, and rational--"before it is united to the body." But no power of the sensitive part belongs to the soul alone, but to the soul and body united, as we have said above (78, 5,8). Therefore the irascible and concupiscible powers are in the will, which is the intellectual appetite.
On the contrary, Gregory of Nyssa (Nemesius, De Nat. Hom.) says "that the irrational" part of the soul is divided into the desiderative and irascible, and Damascene says the same (De Fide Orth. ii, 12). And the Philosopher says (De Anima iii, 9) "that the will is in reason, while in the irrational part of the soul are concupiscence and anger," or "desire and animus."
I answer that, The irascible and concupiscible are not parts of the intellectual appetite, which is called the will. Because, as was said above (59, 4; 79, 7), a power which is directed to an object according to some common notion is not differentiated by special differences which are contained under that common notion. For instance, because sight regards the visible thing under the common notion of something colored, the visual power is not multiplied according to the different kinds of color: but if there were a power regarding white as white, and not as something colored, it would be distinct from a power regarding black as black.
Now the sensitive appetite does not consider the common notion of good, because neither do the senses apprehend the universal. And therefore the parts of the sensitive appetite are differentiated by the different notions of particular good: for the concupiscible regards as proper to it the notion of good, as something pleasant to the senses and suitable to nature: whereas the irascible regards the notion of good as something that wards off and repels what is hurtful. But the will regards good according to the common notion of good, and therefore in the will, which is the intellectual appetite, there is no differentiation of appetitive powers, so that there be in the intellectual appetite an irascible power distinct from a concupiscible power: just as neither on the part of the intellect are the apprehensive powers multiplied, although they are on the part of the senses.
Reply to Objection 1. Love, concupiscence, and the like can be understood in two ways. Sometimes they are taken as passions--arising, that is, with a certain commotion of the soul. And thus they are commonly understood, and in this sense they are only in the sensitive appetite. They may, however, be taken in another way, as far as they are simple affections without passion or commotion of the soul, and thus they are acts of the will. And in this sense, too, they are attributed to the angels and to God. But if taken in this sense, they do not belong to different powers, but only to one power, which is called the will.
Reply to Objection 2. The will itself may be said to irascible, as far as it wills to repel evil, not from any sudden movement of a passion, but from a judgment of the reason. And in the same way the will may be said to be concupiscible on account of its desire for good. And thus in the irascible and concupiscible are charity and hope--that is, in the will as ordered to such acts. And in this way, too, we may understand the words quoted (De Spiritu et Anima); that the irascible and concupiscible powers are in the soul before it is united to the body (as long as we understand priority of nature, and not of time), although there is no need to have faith in what that book says. Whence the answer to the third objection is clear.
Source: Summa Theologica (New Advent)