135. Operative acquired habits are defined as qualities not easily changed, by which a faculty that is able to act in various ways is disposed to act in one way with ease, readiness and pleasure. Thus, by training a man acquires a correct carriage, and is able to walk straight without difficulty.
136. Operative infused habits are enduring qualities that give to a faculty the power to perform acts that are supernatural. Thus, the infused virtues of faith, hope and charity give to the intellect and the will the ability to elicit acts with reference to supernatural truth and good. Facility and promptitude with respect to these acts come through the use of the infused power.
137. Strengthening and Weakening of Habits.—Habits are increased: (a) extensively when they are applied to more objects—thus the habit of science grows as it is applied to more truths; (b) intensively, when they are rooted more firmly in their subject and become easier to exercise. This last comes about when intense acts of a habit are frequently repeated. Thus, a habit of virtue or vice becomes a second nature, and it is exercised with ever greater delight and resisted with ever-increasing difficulty.
138. The infused habits cannot be diminished, but they can be destroyed (see 745). As to the acquired habits, they are weakened and destroyed chiefly in two ways: (a) by acts opposed to them, especially if these acts are earnest and frequent—thus, evil custom is overcome by good custom, and vice-versa; (b) by long discontinuance or disuse. Thus, a person who has learned a foreign language will forget it, if he fails to speak, read or hear it. The knowledge of first principles, speculative or moral, is not lost, however, through forgetfulness, as experience shows.
139. Accidentally, a habit may be corrupted through injury of an organ that is necessary for the exercise of the habit. Thus, right moral judgment may be lost if certain areas of the brain are affected.
140. Habits and Morality.—The importance of habits in man’s moral life is very great. (a) Habits are an index to a man’s past career, for the ease and facility he now possesses through them is the result of many struggles and efforts and difficulties overcome, or of defeats and surrenders and neglected opportunities. (b) Habits constitute a man’s moral character. Morally, a person is the sum of his moral habits and dispositions grouped around the central interest or idea of his life. He who would know himself, therefore, cannot do better than to examine what are his habits, and which is the predominant one among them. (e) Habits are a prophecy of the future. Habits are not irresistible and do not destroy freedom, but they produce such ease and readiness for acting in one particular way that the probabilities are, when habits are strong, that a person will continue to follow them in the future as he has done in the past, thus progressing or deteriorating, as the case may be.
141. Duties as regards Habits.—(a) Bad habits should be avoided and those that have been formed should be destroyed (see 138). The means to accomplish these victories are divine help obtained through prayer and the other instrumentalities of grace, watchfulness through self-examination, and the cultivation of a spirit of self-denial, as well as attack made on the habit that is forming or already formed (see 255 sqq.)
(b) Good habits should be acquired, and those already possessed should be exercised and put to the best advantage. The means to this end, in addition to those that are supernatural, are especially a realization of the importance of good habits, a great desire to have them, and constant and regular effort to practise them (see 137).
Art. 2: GOOD HABITS OR VIRTUES
(_Summa Theologica_, I-II, qq. 55-70.)