Objection 1. It would seem that grace is not in the essence of the soul, as in a subject, but in one of the powers. For Augustine says (Hypognosticon iii [Among the spurious works of St. Augustine]) that grace is related to the will or to the free will "as a rider to his horse." Now the will or the free will is a power, as stated above (I, 83, 2). Hence grace is in a power of the soul, as in a subject.
Objection 2. Further, "Man's merit springs from grace" as Augustine says (De Gratia et Lib. Arbit. vi). Now merit consists in acts, which proceed from a power. Hence it seems that grace is a perfection of a power of the soul.
Objection 3. Further, if the essence of the soul is the proper subject of grace, the soul, inasmuch as it has an essence, must be capable of grace. But this is false; since it would follow that every soul would be capable of grace. Therefore the essence of the soul is not the proper subject of grace.
Objection 4. Further, the essence of the soul is prior to its powers. Now what is prior may be understood without what is posterior. Hence it follows that grace may be taken to be in the soul, although we suppose no part or power of the soul--viz. neither the will, nor the intellect, nor anything else; which is impossible.
On the contrary, By grace we are born again sons of God. But generation terminates at the essence prior to the powers. Therefore grace is in the soul's essence prior to being in the powers.
I answer that, This question depends on the preceding. For if grace is the same as virtue, it must necessarily be in the powers of the soul as in a subject; since the soul's powers are the proper subject of virtue, as stated above (Question 56, Article 1). But if grace differs from virtue, it cannot be said that a power of the soul is the subject of grace, since every perfection of the soul's powers has the nature of virtue, as stated above (55, 1; 56, 1). Hence it remains that grace, as it is prior to virtue, has a subject prior to the powers of the soul, so that it is in the essence of the soul. For as man in his intellective powers participates in the Divine knowledge through the virtue of faith, and in his power of will participates in the Divine love through the virtue of charity, so also in the nature of the soul does he participate in the Divine Nature, after the manner of a likeness, through a certain regeneration or re-creation.
Reply to Objection 1. As from the essence of the soul flows its powers, which are the principles of deeds, so likewise the virtues, whereby the powers are moved to act, flow into the powers of the soul from grace. And thus grace is compared to the will as the mover to the moved, which is the same comparison as that of a horseman to the horse--but not as an accident to a subject.
And thereby is made clear the Reply to the Second Objection. For grace is the principle of meritorious works through the medium of virtues, as the essence of the soul is the principal of vital deeds through the medium of the powers.
Reply to Objection 3. The soul is the subject of grace, as being in the species of intellectual or rational nature. But the soul is not classed in a species by any of its powers, since the powers are natural properties of the soul following upon the species. Hence the soul differs specifically in its essence from other souls, viz. of dumb animals, and of plants. Consequently it does not follow that, if the essence of the human soul is the subject of grace, every soul may be the subject of grace; since it belongs to the essence of the soul, inasmuch as it is of such a species.
Reply to Objection 4. Since the powers of the soul are natural properties following upon the species, the soul cannot be without them. Yet, granted that it was without them, the soul would still be called intellectual or rational in its species, not that it would actually have these powers, but on account of the essence of such a species, from which these powers naturally flow.
Source: Summa Theologica (New Advent)