Objection 1. It would seem that the intellect is not a power of the soul, but the essence of the soul. For the intellect seems to be the same as the mind. Now the mind is not a power of the soul, but the essence; for Augustine says (De Trin. ix, 2): "Mind and spirit are not relative things, but denominate the essence." Therefore the intellect is the essence of the soul.
Objection 2. Further, different genera of the soul's powers are not united in some one power, but only in the essence of the soul. Now the appetitive and the intellectual are different genera of the soul's powers as the Philosopher says (De Anima ii, 3), but they are united in the mind, for Augustine (De Trin. x, 11) places the intelligence and will in the mind. Therefore the mind and intellect of man is of the very essence of the soul and not a power thereof.
Objection 3. Further, according to Gregory, in a homily for the Ascension (xxix in Ev.), "man understands with the angels." But angels are called "minds" and "intellects." Therefore the mind and intellect of man are not a power of the soul, but the soul itself.
Objection 4. Further, a substance is intellectual by the fact that it is immaterial. But the soul is immaterial through its essence. Therefore it seems that the soul must be intellectual through its essence.
On the contrary, The Philosopher assigns the intellectual faculty as a power of the soul (De Anima ii, 3).
I answer that, In accordance with what has been already shown (54, 3; 77, 1) it is necessary to say that the intellect is a power of the soul, and not the very essence of the soul. For then alone the essence of that which operates is the immediate principle of operation, when operation itself is its being: for as power is to operation as its act, so is the essence to being. But in God alone His action of understanding is His very Being. Wherefore in God alone is His intellect His essence: while in other intellectual creatures, the intellect is power.
Reply to Objection 1. Sense is sometimes taken for the power, and sometimes for the sensitive soul; for the sensitive soul takes its name from its chief power, which is sense. And in like manner the intellectual soul is sometimes called intellect, as from its chief power; and thus we read (De Anima i, 4), that the "intellect is a substance." And in this sense also Augustine says that the mind is spirit and essence (De Trin. ix, 2; xiv, 16).
Reply to Objection 2. The appetitive and intellectual powers are different genera of powers in the soul, by reason of the different formalities of their objects. But the appetitive power agrees partly with the intellectual power and partly with the sensitive in its mode of operation either through a corporeal organ or without it: for appetite follows apprehension. And in this way Augustine puts the will in the mind; and the Philosopher, in the reason (De Anima iii, 9).
Reply to Objection 3. In the angels there is no other power besides the intellect, and the will, which follows the intellect. And for this reason an angel is called a "mind" or an "intellect"; because his whole power consists in this. But the soul has many other powers, such as the sensitive and nutritive powers, and therefore the comparison fails.
Reply to Objection 4. The immateriality of the created intelligent substance is not its intellect; and through its immateriality it has the power of intelligence. Wherefore it follows not that the intellect is the substance of the soul, but that it is its virtue and power.
Source: Summa Theologica (New Advent)