Objection 1. It would seem that the intellect does not understand the act of the will. For nothing is known by the intellect, unless it be in some way present in the intellect. But the act of the will is not in the intellect; since the will and the intellect are distinct. Therefore the act of the will is not known by the intellect.
Objection 2. Further, the act is specified by the object. But the object of the will is not the same as the object of the intellect. Therefore the act of the will is specifically distinct from the object of the intellect, and therefore the act of the will is not known by the intellect.
Objection 3. Augustine (Confess. x, 17) says of the soul's affections that "they are known neither by images as bodies are known; nor by their presence, like the arts; but by certain notions." Now it does not seem that there can be in the soul any other notions of things but either the essences of things known or the likenesses thereof. Therefore it seems impossible for the intellect to known such affections of the soul as the acts of the will.
On the contrary, Augustine says (De Trin. x, 11), "I understand that I will."
I answer that, As stated above (Question 59, Article 1), the act of the will is nothing but an inclination consequent on the form understood; just as the natural appetite is an inclination consequent on the natural form. Now the inclination of a thing resides in it according to its mode of existence; and hence the natural inclination resides in a natural thing naturally, and the inclination called the sensible appetite is in the sensible thing sensibly; and likewise the intelligible inclination, which is the act of the will, is in the intelligent subject intelligibly as in its principle and proper subject. Hence the Philosopher expresses himself thus (De Anima iii, 9)--that "the will is in the reason." Now whatever is intelligibly in an intelligent subject, is understood by that subject. Therefore the act of the will is understood by the intellect, both inasmuch as one knows that one wills; and inasmuch as one knows the nature of this act, and consequently, the nature of its principle which is the habit or power.
Reply to Objection 1. This argument would hold good if the will and the intellect were in different subjects, as they are distinct powers; for then whatever was in the will would not be in the intellect. But as both are rooted in the same substance of the soul, and since one is in a certain way the principle of the other, consequently what is in the will is, in a certain way, also in the intellect.
Reply to Objection 2. The "good" and the "true" which are the objects of the will and of the intellect, differ logically, but one is contained in the other, as we have said above (82, 4, ad 1; 16, 4, ad 1); for the true is good and the good is true. Therefore the objects of the will fall under the intellect, and those of the intellect can fall under the will.
Reply to Objection 3. The affections of the soul are in the intellect not by similitude only, like bodies; nor by being present in their subject, as the arts; but as the thing caused is in its principle, which contains some notion of the thing caused. And so Augustine says that the soul's affections are in the memory by certain notions.
Source: Summa Theologica (New Advent)