Objection 1. It would seem that our intellect knows the habits of the soul by their essence. For Augustine says (De Trin. xiii, 1): "Faith is not seen in the heart wherein it abides, as the soul of a man may be seen by another from the movement of the body; but we know most certainly that it is there, and conscience proclaims its existence"; and the same principle applies to the other habits of the soul. Therefore the habits of the soul are not known by their acts, but by themselves.
Objection 2. Further, material things outside the soul are known by their likeness being present in the soul, and are said therefore to be known by their likenesses. But the soul's habits are present by their essence in the soul. Therefore the habits of the soul are known by their essence.
Objection 3. Further, "whatever is the cause of a thing being such is still more so." But habits and intelligible species cause things to be known by the soul. Therefore they are still more known by the soul in themselves.
On the contrary, Habits like powers are the principles of acts. But as is said (De Anima ii, 4), "acts and operations are logically prior to powers." Therefore in the same way they are prior to habits; and thus habits, like the powers, are known by their acts.
I answer that, A habit is a kind of medium between mere power and mere act. Now, it has been said (1) that nothing is known but as it is actual: therefore so far as a habit fails in being a perfect act, it falls short in being of itself knowable, and can be known only by its act; thus, for example, anyone knows he has a habit from the fact that he can produce the act proper to that habit; or he may inquire into the nature and idea of the habit by considering the act. The first kind of knowledge of the habit arises from its being present, for the very fact of its presence causes the act whereby it is known. The second kind of knowledge of the habit arises from a careful inquiry, as is explained above of the mind (1).
Reply to Objection 1. Although faith is not known by external movement of the body, it is perceived by the subject wherein it resides, by the interior act of the heart. For no one knows that he has faith unless he knows that he believes.
Reply to Objection 2. Habits are present in our intellect, not as its object since, in the present state of life, our intellect's object is the nature of a material thing as stated above (Question 84, Article 7), but as that by which it understands.
Reply to Objection 3. The axiom, "whatever is the cause of a thing being such, is still more so," is true of things that are of the same order, for instance, of the same kind of cause; for example, we may say that health is desirable on account of life, and therefore life is more desirable still. But if we take things of different orders the axiom is not true: for we may say that health is caused by medicine, but it does not follow that medicine is more desirable than health, for health belongs to the order of final causes, whereas medicine belongs to the order of efficient causes. So of two things belonging essentially to the order of the objects of knowledge, the one which is the cause of the other being known, is the more known, as principles are more known than conclusions. But habit as such does not belong to the order of objects of knowledge; nor are things known on account of the habit, as on account of an object known, but as on account of a disposition or form whereby the subject knows: and therefore the argument does not prove.
Source: Summa Theologica (New Advent)