Reply Obj. 1:  As stated in Ethic. vii, 10, a habit is like a second nature, and yet it falls short of it.  And so it is that while the nature of a thing cannot in any way be taken away from a thing, a habit is removed, though with difficulty.

Reply Obj. 2:  Although there is no contrary to intelligible species, yet there can be a contrary to assertions and to the process of reason, as stated above.

Reply Obj. 3:  Science is not taken away by movement of the body, if we consider the root itself of the habit, but only as it may prove an obstacle to the act of science; in so far as the intellect, in its act, has need of the sensitive powers, which are impeded by corporal transmutation.  But the intellectual movement of the reason can corrupt the habit of science, even as regards the very root of the habit.  In like manner a habit of virtue can be corrupted.  Nevertheless when it is said that “virtue is more lasting than learning,” this must be understood in respect, not of the subject or cause, but of the act:  because the use of virtue continues through the whole of life, whereas the use of learning does not. ________________________

SECOND ARTICLE [I-II, Q. 53, Art. 2]

Whether a Habit Can Diminish?

Objection 1:  It would seem that a habit cannot diminish.  Because a habit is a simple quality and form.  Now a simple thing is possessed either wholly or not at all.  Therefore although a habit can be lost it cannot diminish.

Obj. 2:  Further, if a thing is befitting an accident, this is by reason either of the accident or of its subject.  Now a habit does not become more or less intense by reason of itself; else it would follow that a species might be predicated of its individuals more or less.  And if it can become less intense as to its participation by its subject, it would follow that something is accidental to a habit, proper thereto and not common to the habit and its subject.  Now whenever a form has something proper to it besides its subject, that form can be separate, as stated in De Anima i, text. 13.  Hence it follows that a habit is a separable form; which is impossible.

Obj. 3:  Further, the very notion and nature of a habit as of any accident, is inherence in a subject:  wherefore any accident is defined with reference to its subject.  Therefore if a habit does not become more or less intense in itself, neither can it in its inherence in its subject:  and consequently it will be nowise less intense.

On the contrary, It is natural for contraries to be applicable to the same thing.  Now increase and decrease are contraries.  Since therefore a habit can increase, it seems that it can also diminish.

I answer that, Habits diminish, just as they increase, in two ways, as we have already explained (Q. 52, A. 1).  And since they increase through the same cause as that which engenders them, so too they diminish by the same cause as that which corrupts them:  since the diminishing of a habit is the road which leads to its corruption, even as, on the other hand, the engendering of a habit is a foundation of its increase.