Objection 1. It would seem that a habit is not corrupted or diminished through mere cessation from act. For habits are more lasting than passion-like qualities, as we have explained above (49, 2, ad 3; 50, 1). But passion-like qualities are neither corrupted nor diminished by cessation from act: for whiteness is not lessened through not affecting the sight, nor heat through ceasing to make something hot. Therefore neither are habits diminished or corrupted through cessation from act.
Objection 2. Further, corruption and diminution are changes. Now nothing is changed without a moving cause. Since therefore cessation from act does not imply a moving cause, it does not appear how a habit can be diminished or corrupted through cessation from act.
Objection 3. Further, the habits of science and virtue are in the intellectual soul which is above time. Now those things that are above time are neither destroyed nor diminished by length of time. Neither, therefore, are such habits destroyed or diminished through length of time, if one fails for long to exercise them.
On the contrary, The Philosopher says (De Long. et Brev. Vitae ii) that not only "deception," but also "forgetfulness, is the corruption of science." Moreover he says (Ethic. viii, 5) that "want of intercourse has dissolved many a friendship." In like manner other habits of virtue are diminished or destroyed through cessation from act.
I answer that, As stated in Phys. vii, text. 27, a thing is a cause of movement in two ways. First, directly; and such a thing causes movement by reason of its proper form; thus fire causes heat. Secondly, indirectly; for instance, that which removes an obstacle. It is in this latter way that the destruction or diminution of a habit results through cessation from act, in so far, to wit, as we cease from exercising an act which overcame the causes that destroyed or weakened that habit. For it has been stated (1) that habits are destroyed or diminished directly through some contrary agency. Consequently all habits that are gradually undermined by contrary agencies which need to be counteracted by acts proceeding from those habits, are diminished or even destroyed altogether by long cessation from act, as is clearly seen in the case both of science and of virtue. For it is evident that a habit of moral virtue makes a man ready to choose the mean in deeds and passions. And when a man fails to make use of his virtuous habit in order to moderate his own passions or deeds, the necessary result is that many passions and deeds fail to observe the mode of virtue, by reason of the inclination of the sensitive appetite and of other external agencies. Wherefore virtue is destroyed or lessened through cessation from act. The same applies to the intellectual habits, which render man ready to judge aright of those things that are pictured by his imagination. Hence when man ceases to make use of his intellectual habits, strange fancies, sometimes in opposition to them, arise in his imagination; so that unless those fancies be, as it were, cut off or kept back by frequent use of his intellectual habits, man becomes less fit to judge aright, and sometimes is even wholly disposed to the contrary, and thus the intellectual habit is diminished or even wholly destroyed by cessation from act.
Reply to Objection 1. Even heat would be destroyed through ceasing to give heat, if, for this same reason, cold which is destructive of heat were to increase.
Reply to Objection 2. Cessation from act is a moving cause, conducive of corruption or diminution, by removing the obstacles, thereto, as explained above.
Reply to Objection 3. The intellectual part of the soul, considered in itself, is above time, but the sensitive part is subject to time, and therefore in course of time it undergoes change as to the passions of the sensitive part, and also as to the powers of apprehension. Hence the Philosopher says (Phys. iv. text. 117) that time makes us forget.
Source: Summa Theologica (New Advent)