Objection 1. It would seem that original sin is not in the essence of the soul rather than in the powers. For the soul is naturally apt to be the subject of sin, in respect of those parts which can be moved by the will. Now the soul is moved by the will, not as to its essence but only as to the powers. Therefore original sin is in the soul, not according to its essence, but only according to the powers.
Objection 2. Further, original sin is opposed to original justice. Now original justice was in a power of the soul, because power is the subject of virtue. Therefore original sin also is in a power of the soul, rather than in its essence.
Objection 3. Further, just as original sin is derived from the soul as from the flesh, so is it derived by the powers from the essence. But original sin is more in the soul than in the flesh. Therefore it is more in the powers than in the essence of the soul.
Objection 4. Further, original sin is said to be concupiscence, as stated (82, 3). But concupiscence is in the powers of the soul. Therefore original sin is also.
On the contrary, Original sin is called the sin of nature, as stated above (Question 81, Article 1). Now the soul is the form and nature of the body, in respect of its essence and not in respect of its powers, as stated in the I, 76, 6. Therefore the soul is the subject of original sin chiefly in respect of its essence.
I answer that, The subject of a sin is chiefly that part of the soul to which the motive cause of that sin primarily pertains: thus if the motive cause of a sin is sensual pleasure, which regards the concupiscible power through being its proper object, it follows that the concupiscible power is the proper subject of that sin. Now it is evident that original sin is caused through our origin. Consequently that part of the soul which is first reached by man's origin, is the primary subject of original sin. Now the origin reaches the soul as the term of generation, according as it is the form of the body: and this belongs to the soul in respect of its essence, as was proved in the I, 76, 6. Therefore the soul, in respect of its essence, is the primary subject of original sin.
Reply to Objection 1. As the motion of the will of an individual reaches to the soul's powers and not to its essence, so the motion of the will of the first generator, through the channel of generation, reaches first of all to the essence of the soul, as stated.
Reply to Objection 2. Even original justice pertained radically to the essence of the soul, because it was God's gift to human nature, to which the essence of the soul is related before the powers. For the powers seem to regard the person, in as much as they are the principles of personal acts. Hence they are the proper subjects of actual sins, which are the sins of the person.
Reply to Objection 3. The body is related to the soul as matter to form, which though it comes second in order of generation, nevertheless comes first in the order of perfection and nature. But the essence of the soul is related to the powers, as a subject to its proper accidents, which follow their subject both in the order of generation and in that of perfection. Consequently the comparison fails.
Reply to Objection 4. Concupiscence, in relation to original sin, holds the position of matter and effect, as stated above (Question 82, Article 3).
Source: Summa Theologica (New Advent)