of the increase of habits as we do of the increase of an animal.  For not every morsel of food actually increases the animal’s size as neither does every drop of water hollow out the stone:  but the multiplication of food results at last in an increase of the body.  So, too, repeated acts cause a habit to grow.  If, however, the act falls short of the intensity of the habit, such an act does not dispose to an increase of that habit, but rather to a lessening thereof.

From this it is clear how to solve the objections.
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QUESTION 53

HOW HABITS ARE CORRUPTED OR DIMINISHED
(In Three Articles)

We must now consider how habits are lost or weakened; and under this head there are three points of inquiry: 

(1) Whether a habit can be corrupted?

(2) Whether it can be diminished?

(3) How are habits corrupted or diminished?
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FIRST ARTICLE [I-II, Q. 53, Art. 1]

Whether a Habit Can Be Corrupted?

Objection 1:  It would seem that a habit cannot be corrupted.  For habit is within its subject like a second nature; wherefore it is pleasant to act from habit.  Now so long as a thing is, its nature is not corrupted.  Therefore neither can a habit be corrupted so long as its subject remains.

Obj. 2:  Further, whenever a form is corrupted, this is due either to corruption of its subject, or to its contrary:  thus sickness ceases through corruption of the animal, or through the advent of health.  Now science, which is a habit, cannot be lost through corruption of its subject:  since “the intellect,” which is its subject, “is a substance that is incorruptible” (De Anima i, text. 65).  In like manner, neither can it be lost through the action of its contrary:  since intelligible species are not contrary to one another (Metaph. vii, text. 52).  Therefore the habit of science can nowise be lost.

Obj. 3:  Further, all corruption results from some movement.  But the habit of science, which is in the soul, cannot be corrupted by a direct movement of the soul itself, since the soul is not moved directly.  It is, however, moved indirectly through the movement of the body:  and yet no bodily change seems capable of corrupting the intelligible species residing in the intellect:  since the intellect independently of the body is the proper abode of the species; for which reason it is held that habits are not lost either through old age or through death.  Therefore science cannot be corrupted.  For the same reason neither can habits of virtue be corrupted, since they also are in the rational soul, and, as the Philosopher declares (Ethic. i, 10), “virtue is more lasting than learning.”

On the contrary, The Philosopher says (De Long. et Brev.  Vitae ii) that “forgetfulness and deception are the corruption of science.”  Moreover, by sinning a man loses a habit of virtue:  and again, virtues are engendered and corrupted by contrary acts (Ethic. ii, 2).