(a) From the viewpoint of their subject, they are either entitative or operative, according as they affect directly the nature or the powers of a being. Thus, in the soul there are the entitative habit of sanctifying grace and operative habits like science and virtue; while in the body are entitative habits of health, beauty, etc. (b) From the viewpoint of their object, habits are good (i.e., virtues) or evil (i.e., vices); (c) From the viewpoint of their cause, habits are infused or acquired, according as they are supernaturally produced by God, or are naturally obtained by man through repeated acts, or result from nature without repeated acts. Faith in a baptized infant is an infused habit; knowledge obtained through study is an acquired habit; the perception that the first principles of truth are to be granted is natural. 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.