Objection 1. It would seem that one virtue can be in several powers. For habits are known by their acts. But one act proceeds in various way from several powers: thus walking proceeds from the reason as directing, from the will as moving, and from the motive power as executing. Therefore also one habit can be in several powers.
Objection 2. Further, the Philosopher says (Ethic. ii, 4) that three things are required for virtue, namely: "to know, to will, and to work steadfastly." But "to know" belongs to the intellect, and "to will" belongs to the will. Therefore virtue can be in several powers.
Objection 3. Further, prudence is in the reason since it is "the right reason of things to be done" (Ethic. vi, 5). And it is also in the will: for it cannot exist together with a perverse will (Ethic. vi, 12). Therefore one virtue can be in two powers.
On the contrary, The subject of virtue is a power of the soul. But the same accident cannot be in several subjects. Therefore one virtue cannot be in several powers of the soul.
I answer that, It happens in two ways that one thing is subjected in two. First, so that it is in both on an equal footing. In this way it is impossible for one virtue to be in two powers: since diversity of powers follows the generic conditions of the objects, while diversity of habits follows the specific conditions thereof: and so wherever there is diversity of powers, there is diversity of habits; but not vice versa. In another way one thing can be subjected in two or more, not on an equal footing, but in a certain order. And thus one virtue can belong to several powers, so that it is in one chiefly, while it extends to others by a kind of diffusion, or by way of a disposition, in so far as one power is moved by another, and one power receives from another.
Reply to Objection 1. One act cannot belong to several powers equally, and in the same degree; but only from different points of view, and in various degrees.
Reply to Objection 2. "To know" is a condition required for moral virtue, inasmuch as moral virtue works according to right reason. But moral virtue is essentially in the appetite.
Reply to Objection 3. Prudence is really subjected in reason: but it presupposes as its principle the rectitude of the will, as we shall see further on (3; 57, 4).
Source: Summa Theologica (New Advent)