Objection 1. It would seem that there are no habits in the intellect. For habits are in conformity with operations, as stated above (Article 1). But the operations of man are common to soul and body, as stated in De Anima i, text. 64. Therefore also are habits. But the intellect is not an act of the body (De Anima iii, text. 6). Therefore the intellect is not the subject of a habit.
Objection 2. Further, whatever is in a thing, is there according to the mode of that in which it is. But that which is form without matter, is act only: whereas what is composed of form and matter, has potentiality and act at the same time. Therefore nothing at the same time potential and actual can be in that which is form only, but only in that which is composed of matter and form. Now the intellect is form without matter. Therefore habit, which has potentiality at the same time as act, being a sort of medium between the two, cannot be in the intellect; but only in the "conjunction," which is composed of soul and body.
Objection 3. Further, habit is a disposition whereby we are well or ill disposed in regard to something, as is said (Metaph. v, text. 25). But that anyone should be well or ill disposed to an act of the intellect is due to some disposition of the body: wherefore also it is stated (De Anima ii, text. 94) that "we observe men with soft flesh to be quick witted." Therefore the habits of knowledge are not in the intellect, which is separate, but in some power which is the act of some part of the body.
On the contrary, The Philosopher (Ethic. vi, 2,3,10) puts science, wisdom and understanding, which is the habit of first principles, in the intellective part of the soul.
I answer that, concerning intellective habits there have been various opinions. Some, supposing that there was only one "possible" [I, 79, 2, ad 2] intellect for all men, were bound to hold that habits of knowledge are not in the intellect itself, but in the interior sensitive powers. For it is manifest that men differ in habits; and so it was impossible to put the habits of knowledge directly in that, which, being only one, would be common to all men. Wherefore if there were but one single "possible" intellect of all men, the habits of science, in which men differ from one another, could not be in the "possible" intellect as their subject, but would be in the interior sensitive powers, which differ in various men.
Now, in the first place, this supposition is contrary to the mind of Aristotle. For it is manifest that the sensitive powers are rational, not by their essence, but only by participation (Ethic. i, 13). Now the Philosopher puts the intellectual virtues, which are wisdom, science and understanding, in that which is rational by its essence. Wherefore they are not in the sensitive powers, but in the intellect itself. Moreover he says expressly (De Anima iii, text. 8,18) that when the "possible" intellect "is thus identified with each thing," that is, when it is reduced to act in respect of singulars by the intelligible species, "then it is said to be in act, as the knower is said to be in act; and this happens when the intellect can act of itself," i.e. by considering: "and even then it is in potentiality in a sense; but not in the same way as before learning and discovering." Therefore the "possible" intellect itself is the subject of the habit of science, by which the intellect, even though it be not actually considering, is able to consider. In the second place, this supposition is contrary to the truth. For as to whom belongs the operation, belongs also the power to operate, belongs also the habit. But to understand and to consider is the proper act of the intellect. Therefore also the habit whereby one considers is properly in the intellect itself.
Reply to Objection 1. Some said, as Simplicius reports in his Commentary on the Predicaments, that, since every operation of man is to a certain extent an operation of the "conjunctum," as the Philosopher says (De Anima i, text. 64); therefore no habit is in the soul only, but in the "conjunctum." And from this it follows that no habit is in the intellect, for the intellect is separate, as ran the argument, given above. But the argument is no cogent. For habit is not a disposition of the object to the power, but rather a disposition of the power to the object: wherefore the habit needs to be in that power which is principle of the act, and not in that which is compared to the power as its object.
Now the act of understanding is not said to be common to soul and body, except in respect of the phantasm, as is stated in De Anima, text. 66. But it is clear that the phantasm is compared as object to the passive intellect (De Anima iii, text. 3,39). Whence it follows that the intellective habit is chiefly on the part of the intellect itself; and not on the part of the phantasm, which is common to soul and body. And therefore we must say that the "possible" intellect is the subject of habit, which is in potentiality to many: and this belongs, above all, to the "possible" intellect. Wherefore the "possible" intellect is the subject of intellectual habits.
Reply to Objection 2. As potentiality to sensible being belongs to corporeal matter, so potentiality to intellectual being belongs to the "possible" intellect. Wherefore nothing forbids habit to be in the "possible" intellect, for it is midway between pure potentiality and perfect act.
Reply to Objection 3. Because the apprehensive powers inwardly prepare their proper objects for the "possible intellect," therefore it is by the good disposition of these powers, to which the good disposition of the body cooperates, that man is rendered apt to understand. And so in a secondary way the intellective habit can be in these powers. But principally it is in the "possible" intellect.
Source: Summa Theologica (New Advent)