Summa Theologica, by St. Thomas Aquinas, , at sacred-texts.com
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?
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.
Objection 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.
Objection 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).
I answer that, A form is said to be corrupted directly by its contrary; indirectly, through its subject being corrupted. When therefore a habit has a corruptible subject, and a cause that has a contrary, it can be corrupted both ways. This is clearly the case with bodily habits---for instance, health and sickness. But those habits that have an incorruptible subject, cannot be corrupted indirectly. There are, however, some habits which, while residing chiefly in an incorruptible subject, reside nevertheless secondarily in a corruptible subject; such is the habit of science which is chiefly indeed in the "possible" intellect, but secondarily in the sensitive powers of apprehension, as stated above (Q, A, ad 3). Consequently the habit of science cannot be corrupted indirectly, on the part of the "possible" intellect, but only on the part of the lower sensitive powers.
We must therefore inquire whether habits of this kind can be corrupted directly. If then there be a habit having a contrary, either on the part of itself or on the part of its cause, it can be corrupted directly: but if it has no contrary, it cannot be corrupted directly. Now it is evident that an intelligible species residing in the "possible" intellect, has no contrary; nor can the active intellect, which is the cause of that species, have a contrary. Wherefore if in the "possible" intellect there be a habit caused immediately by the active intellect, such a habit is incorruptible both directly and indirectly. Such are the habits of the first principles, both speculative and practical, which cannot be corrupted by any forgetfulness or deception whatever: even as the Philosopher says about prudence (Ethic. vi, 5) that "it cannot be lost by being forgotten." There is, however, in the "possible" intellect a habit caused by the reason, to wit, the habit of conclusions, which is called science, to the cause of which something may be contrary in two ways. First, on the part of those very propositions which are the starting point of the reason: for the assertion "Good is not good" is contrary to the assertion "Good is good" (Peri Herm. ii). Secondly, on the part of the process of reasoning; forasmuch as a sophistical syllogism is contrary to a dialectic or demonstrative syllogism. Wherefore it is clear that a false reason can corrupt the habit of a true opinion or even of science. Hence the Philosopher, as stated above, says that "deception is the corruption of science." As to virtues, some of them are intellectual, residing in reason itself, as stated in Ethic. vi, 1: and to these applies what we have said of science and opinion. Some, however, viz. the moral virtues, are in the appetitive part of the soul; and the same may be said of the contrary vices. Now the habits of the appetitive part are caused therein because it is natural to it to be moved by the reason. Therefore a habit either of virtue or of vice, may be corrupted by a judgment of reason, whenever its motion is contrary to such vice or virtue, whether through ignorance, passion or deliberate choice.
Reply to Objection 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 to Objection 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 to Objection 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.
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.
Objection 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.
Objection 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, A). 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.
Reply to Objection 1: A habit, considered in itself, is a simple form. It is not thus that it is subject to decrease; but according to the different ways in which its subject participates in it. This is due to the fact that the subject's potentiality is indeterminate, through its being able to participate a form in various ways, or to extend to a greater or a smaller number of things.
Reply to Objection 2: This argument would hold, if the essence itself of a habit were nowise subject to decrease. This we do not say; but that a certain decrease in the essence of a habit has its origin, not in the habit, but in its subject.
Reply to Objection 3: No matter how we take an accident, its very notion implies dependence on a subject, but in different ways. For if we take an accident in the abstract, it implies relation to a subject, which relation begins in the accident and terminates in the subject: for "whiteness is that whereby a thing is white." Accordingly in defining an accident in the abstract, we do not put the subject as though it were the first part of the definition, viz. the genus; but we give it the second place, which is that of the difference; thus we say that "simitas" is "a curvature of the nose." But if we take accidents in the concrete, the relation begins in the subject and terminates in the concrete, the relation begins in the subject and terminates at the accident: for "a white thing" is "something that has whiteness." Accordingly in defining this kind of accident, we place the subject as the genus, which is the first part of a definition; for we say that a "simum" is a "snub-nose." Accordingly whatever is befitting an accident on the part of the subject, but is not of the very essence of the accident, is ascribed to that accident, not in the abstract, but in the concrete. Such are increase and decrease in certain accidents: wherefore to be more or less white is not ascribed to whiteness but to a white thing. The same applies to habits and other qualities; save that certain habits and other qualities; save that certain habits increase or diminish by a kind of addition, as we have already clearly explained (Q, A).
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 (Q, A, ad 3; Q, A). 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 (A) 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.