Objection 1. It would seem that there is no order among the powers of the soul. For in those things which come under one division, there is no before and after, but all are naturally simultaneous. But the powers of the soul are contradistinguished from one another. Therefore there is no order among them.
Objection 2. Further, the powers of the soul are referred to their objects and to the soul itself. On the part of the soul, there is not order among them, because the soul is one. In like manner the objects are various and dissimilar, as color and sound. Therefore there is no order among the powers of the soul.
Objection 3. Further, where there is order among powers, we find that the operation of one depends on the operation of another. But the action of one power of the soul does not depend on that of another; for sight can act independently of hearing, and conversely. Therefore there is no order among the powers of the soul.
On the contrary, The Philosopher (De Anima ii, 3) compares the parts or powers of the soul to figures. But figures have an order among themselves. Therefore the powers of the soul have order.
I answer that, Since the soul is one, and the powers are many; and since a number of things that proceed from one must proceed in a certain order; there must be some order among the powers of the soul. Accordingly we may observe a triple order among them, two of which correspond to the dependence of one power on another; while the third is taken from the order of the objects. Now the dependence of one power on another can be taken in two ways; according to the order of nature, forasmuch as perfect things are by their nature prior to imperfect things; and according to the order of generation and time; forasmuch as from being imperfect, a thing comes to be perfect. Thus, according to the first kind of order among the powers, the intellectual powers are prior to the sensitive powers; wherefore they direct them and command them. Likewise the sensitive powers are prior in this order to the powers of the nutritive soul.
In the second kind of order, it is the other way about. For the powers of the nutritive soul are prior by way of generation to the powers of the sensitive soul; for which, therefore, they prepare the body. The same is to be said of the sensitive powers with regard to the intellectual. But in the third kind of order, certain sensitive powers are ordered among themselves, namely, sight, hearing, and smelling. For the visible naturally comes first; since it is common to higher and lower bodies. But sound is audible in the air, which is naturally prior to the mingling of elements, of which smell is the result.
Reply to Objection 1. The species of a given genus are to one another as before and after, like numbers and figures, if considered in their nature; although they may be said to be simultaneous, according as they receive the predication of the common genus.
Reply to Objection 2. This order among the powers of the soul is both on the part of the soul (which, though it be one according to its essence, has a certain aptitude to various acts in a certain order) and on the part of the objects, and furthermore on the part of the acts, as we have said above.
Reply to Objection 3. This argument is verified as regards those powers among which order of the third kind exists. Those powers among which the two other kinds of order exist are such that the action of one depends on another.
Source: Summa Theologica (New Advent)