Objection 1. It would seem that the appetite is not a special power of the soul. For no power of the soul is to be assigned for those things which are common to animate and to inanimate things. But appetite is common to animate and inanimate things: since "all desire good," as the Philosopher says (Ethic. i, 1). Therefore the appetite is not a special power of the soul.
Objection 2. Further, powers are differentiated by their objects. But what we desire is the same as what we know. Therefore the appetitive power is not distinct from the apprehensive power.
Objection 3. Further, the common is not divided from the proper. But each power of the soul desires some particular desirable thing--namely its own suitable object. Therefore, with regard to this object which is the desirable in general, we should not assign some particular power distinct from the others, called the appetitive power.
On the contrary, The Philosopher distinguishes (De Anima ii, 3) the appetitive from the other powers. Damascene also (De Fide Orth. ii, 22) distinguishes the appetitive from the cognitive powers.
I answer that, It is necessary to assign an appetitive power to the soul. To make this evident, we must observe that some inclination follows every form: for example, fire, by its form, is inclined to rise, and to generate its like. Now, the form is found to have a more perfect existence in those things which participate knowledge than in those which lack knowledge. For in those which lack knowledge, the form is found to determine each thing only to its own being--that is, to its nature. Therefore this natural form is followed by a natural inclination, which is called the natural appetite. But in those things which have knowledge, each one is determined to its own natural being by its natural form, in such a manner that it is nevertheless receptive of the species of other things: for example, sense receives the species of all things sensible, and the intellect, of all things intelligible, so that the soul of man is, in a way, all things by sense and intellect: and thereby, those things that have knowledge, in a way, approach to a likeness to God, "in Whom all things pre-exist," as Dionysius says (Div. Nom. v).
Therefore, as forms exist in those things that have knowledge in a higher manner and above the manner of natural forms; so must there be in them an inclination surpassing the natural inclination, which is called the natural appetite. And this superior inclination belongs to the appetitive power of the soul, through which the animal is able to desire what it apprehends, and not only that to which it is inclined by its natural form. And so it is necessary to assign an appetitive power to the soul.
Reply to Objection 1. Appetite is found in things which have knowledge, above the common manner in which it is found in all things, as we have said above. Therefore it is necessary to assign to the soul a particular power.
Reply to Objection 2. What is apprehended and what is desired are the same in reality, but differ in aspect: for a thing is apprehended as something sensible or intelligible, whereas it is desired as suitable or good. Now, it is diversity of aspect in the objects, and not material diversity, which demands a diversity of powers.
Reply to Objection 3. Each power of the soul is a form or nature, and has a natural inclination to something. Wherefore each power desires by the natural appetite that object which is suitable to itself. Above which natural appetite is the animal appetite, which follows the apprehension, and by which something is desired not as suitable to this or that power, such as sight for seeing, or sound for hearing; but simply as suitable to the animal.
Source: Summa Theologica (New Advent)