Reply Obj. 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 Obj. 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 Obj. 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. _______________________
FIFTH ARTICLE [I, Q. 77, Art. 5]
Whether All the Powers of the Soul Are in the Soul As Their Subject?
Objection 1: It would seem that all the powers of the soul are in the soul as their subject. For as the powers of the body are to the body; so are the powers of the soul to the soul. But the body is the subject of the corporeal powers. Therefore the soul is the subject of the powers of the soul.
Obj. 2: Further, the operations of the powers of the soul are attributed to the body by reason of the soul; because, as the Philosopher says (De Anima ii, 2), "The soul is that by which we sense and understand primarily." But the natural principles of the operations of the soul are the powers. Therefore the powers are primarily in the soul.
Obj. 3: Further, Augustine says (Gen. ad lit. xii, 7,24) that the soul senses certain things, not through the body, in fact, without the body, as fear and such like; and some things through the body. But if the sensitive powers were not in the soul alone as their subject, the soul could not sense anything without the body. Therefore the soul is the subject of the sensitive powers; and for a similar reason, of all the other powers.
On the contrary, The Philosopher says (De Somno et Vigilia i) that "sensation belongs neither to the soul, nor to the body, but to the composite." Therefore the sensitive power is in "the composite" as its subject. Therefore the soul alone is not the subject of all the powers.
I answer that, The subject of operative power is that which is able to operate, for every accident denominates its proper subject. Now the same is that which is able to operate, and that which does operate. Wherefore the "subject of power" is of necessity "the subject of operation," as again the Philosopher says in the beginning of De Somno et Vigilia. Now, it is clear from what we have said above (Q. 75, AA. 2, 3; Q. 76, A. 1, ad 1), that some operations of the soul are performed without a corporeal organ, as understanding and will. Hence the powers of these operations are in the soul as their subject. But some operations of the soul are performed by means of corporeal organs; as sight by the eye, and hearing by the ear. And so it is with all the other operations of the nutritive and sensitive parts. Therefore the powers which are the principles of these operations have their subject in the composite, and not in the soul alone.