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.
Objection 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.
Objection 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 (75, 2,3; 76, 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.
Reply to Objection 1. All the powers are said to belong to the soul, not as their subject, but as their principle; because it is by the soul that the composite has the power to perform such operations.
Reply to Objection 2. All such powers are primarily in the soul, as compared to the composite; not as in their subject, but as in their principle.
Reply to Objection 3. Plato's opinion was that sensation is an operation proper to the soul, just as understanding is. Now in many things relating to Philosophy Augustine makes use of the opinions of Plato, not asserting them as true, but relating them. However, as far as the present question is concerned, when it is said that the soul senses some things with the body, and some without the body, this can be taken in two ways.
Firstly, the words "with the body or without the body" may determine the act of sense in its mode of proceeding from the sentient. Thus the soul senses nothing without the body, because the action of sensation cannot proceed from the soul except by a corporeal organ.
Secondly, they may be understood as determining the act of sense on the part of the object sensed. Thus the soul senses some things with the body, that is, things existing in the body, as when it feels a wound or something of that sort; while it senses some things without the body, that is, which do not exist in the body, but only in the apprehension of the soul, as when it feels sad or joyful on hearing something.
Source: Summa Theologica (New Advent)