Objection 1. It would seem that habit is in the soul in respect of its essence rather than in respect of its powers. For we speak of dispositions and habits in relation to nature, as stated above (Question 49, Article 2). But nature regards the essence of the soul rather than the powers; because it is in respect of its essence that the soul is the nature of such a body and the form thereof. Therefore habits are in the soul in respect of its essence and not in respect of its powers.
Objection 2. Further, accident is not the subject of accident. Now habit is an accident. But the powers of the soul are in the genus of accident, as we have said in the I, 77, 1, ad 5. Therefore habit is not in the soul in respect of its powers.
Objection 3. Further, the subject is prior to that which is in the subject. But since habit belongs to the first species of quality, it is prior to power, which belongs to the second species. Therefore habit is not in a power of the soul as its subject.
On the contrary, The Philosopher (Ethic. i, 13) puts various habits in the various powers of the soul.
I answer that, As we have said above (49, A2,3), habit implies a certain disposition in relation to nature or to operation. If therefore we take habit as having a relation to nature, it cannot be in the soul--that is, if we speak of human nature: for the soul itself is the form completing the human nature; so that, regarded in this way, habit or disposition is rather to be found in the body by reason of its relation to the soul, than in the soul by reason of its relation to the body. But if we speak of a higher nature, of which man may become a partaker, according to 2 Peter 1, "that we may be partakers of the Divine Nature": thus nothing hinders some habit, namely, grace, from being in the soul in respect of its essence, as we shall state later on (110, 4).
On the other hand, if we take habit in its relation to operation, it is chiefly thus that habits are found in the soul: in so far as the soul is not determined to one operation, but is indifferent to many, which is a condition for a habit, as we have said above (Question 49, Article 4). And since the soul is the principle of operation through its powers, therefore, regarded in this sense, habits are in the soul in respect of its powers.
Reply to Objection 1. The essence of the soul belongs to human nature, not as a subject requiring to be disposed to something further, but as a form and nature to which someone is disposed.
Reply to Objection 2. Accident is not of itself the subject of accident. But since among accidents themselves there is a certain order, the subject, according as it is under one accident, is conceived as the subject of a further accident. In this way we say that one accident is the subject of another; as superficies is the subject of color, in which sense power is the subject of habit.
Reply to Objection 3. Habit takes precedence of power, according as it implies a disposition to nature: whereas power always implies a relation to operation, which is posterior, since nature is the principle of operation. But the habit whose subject is a power, does not imply relation to nature, but to operation. Wherefore it is posterior to power. Or, we may say that habit takes precedence of power, as the complete takes precedence of the incomplete, and as act takes precedence of potentiality. For act is naturally prior to potentiality, though potentiality is prior in order of generation and time, as stated in Metaph. vii, text. 17; ix, text. 13.
Source: Summa Theologica (New Advent)