1. A habit is a quality difficult to change, whereby an agent whose nature it was to work one way or another indeterminately, is disposed easily and readily at will to follow this or that particular line of action. Habit differs from disposition, as disposition is a quality easily changed. Thus one in a good humour is in a disposition to be kind. Habit is a part of character: disposition is a passing fit. Again, habit differs from faculty, or power: as power enables one to act; but habit, presupposing power, renders action easy and expeditious, and reliable to come at call. We have a power to move our limbs, but a habit to walk or ride or swim. Habit then is the determinant of power. One and the same power works well or ill, but not one and the same habit.
2. A power that has only one way of working, set and fixed, is not susceptible of habit. Such powers are the forces of inanimate nature, as gravitation and electricity. A thing does not gravitate better for gravitating often. The moon does not obey the earth more readily to-day than she did in the days of Ptolemy, or of the Chaldean sages. Some specious claim to habit might be set up on behalf of electricity and magnetism. A glass rod rubbed at frequent intervals for six months, is a different instrument from what it would have been, if left all that time idle in a drawer. Then there are such cases as the gradual magnetising of an iron bar. Still we cannot speak of electrical habits, or magnetic habits, not at least in things without life, because there is no will there to control the exercise of the quality. As well might we speak of a "tumbledown" habit in a row of houses, brought on by locomotives running underneath their foundations. It is but a case of an accumulation of small effects, inducing gradually a new molecular arrangement, so that the old powers act under new material conditions. But habit is a thing of life, an appurtenance of will, not of course independent of material conditions and structural alterations, in so far forth as a living and volitional is also a material agent, but essentially usable at will, and brought into play and controlled in its operation by free choice. Therefore a habit that works almost automatically has less of the character of a true habit, and passes rather out of morality into the region of physics. Again, bad habits, vices to which a man is become a slave against his better judgment, are less properly called habits than virtues are; for such evil habits do not so much attend on volition (albeit volition has created them) as drag the will in their wake. For the like reason, habit is less properly predicable of brute animals than of men: for brutes have no intelligent will to govern their habits. The highest brutes are most susceptible of habit. They are most like men in being most educable. And, of human progeny, some take up habits, in the best and completest sense of the term, more readily than others. They are better subjects for education: education being nothing else than the formation of habits.
3. Knowledge consists of intellectual habits. But the habits of most consequence to the moralist lie in the will, and in the sensitive appetite as amenable to the control of the will. In this category come the virtues, in the ordinary sense of that name, and secondarily the vices.
4. A habit is acquired by acts. Whereupon this difficulty has been started:—If the habit, say of mental application, comes from acts of study, and again the acts from the habit, how ever is the habit originally acquired? We answer that there are two ways in which one thing may come from another. It may come in point of its very existence, as child from parent; or in point of some mode of existence, as scholar from master. A habit has its very existence from acts preceding: but those acts have their existence independent of the habit. The acts which are elicited after the habit is formed, owe to the habit, not their existence, but the mode of their existence: that is to say, because of the habit the acts are now formed readily, reliably, and artistically, or virtuously. The primitive acts which gradually engendered the habit, were done with difficulty, fitfully, and with many failures,—more by good luck than good management, if it was a matter of skill, and by a special effort rather than as a thing of course, where it was question of moral well-doing. (See c.ii., s.ii., n.9, p. 10.)
5. A habit is a living thing: it grows and must be fed. It grows on acts, and acts are the food that sustain it. Unexercised, a habit pines away: corruption sets in and disintegration. A man, we will say, has a habit of thinking of God during his work. He gives over doing so. That means that he either takes to thinking of everything and nothing, or he takes up some definite line of thought to the exclusion of God. Either way there is a new formation to the gradual ruin of the old habit.
6. Habit and custom may be distinguished in philosophical language. We may say that custom makes the habit. Custom does not imply any skill or special facility. A habit is a channel whereby the energies flow, as otherwise they would not have flowed, freely and readily in some particular direction. A habit, then, is a determination of a faculty for good or for evil. It is something intrinsic in a man, a real modification of his being, abiding in him in the intervals between one occasion for its exercise and another: whereas custom is a mere denomination, expressive of frequent action and no more. Thus it would be more philosophical to speak of a custom of early rising, and of a custom of smoking, rather than of a habit of smoking, except so far as, by the use of the word habit, you may wish to point to a certain acquired skill of the respiratory and facial muscles, and a certain acquired temper of the stomach, enabling one to inhale tobacco fumes with impunity.
7. Habits are acquired, but it is obvious that the rate of acquisition varies in different persons. This comes from one person being more predisposed by nature than another to the acquiring of this or that habit. By nature, that is by the native temper and conformation of his body wherewith he was born, this child is more prone to literary learning, that to mechanics, this one to obstinacy and contentiousness, that to sensuality, and so of the rest. For though it is by the soul that a man learns, and by the act of his will and spiritual powers he becomes a glutton or a zealot, nevertheless the bodily organs concur and act jointly towards these ends. The native dispositions of the child's body for the acquisition of habits depend to an unascertained extent upon the habits of his ancestors. This is the fact of heredity.
8. Man is said to be "a creature of habits." The formation of habits in the will saves the necessity of continually making up the mind anew. A man will act as he has become habituated, except under some special motive from without, or some special effort from within. In the case of evil habits, that effort is attended with immense difficulty. The habit is indeed the man's own creation, the outcome of his free acts. But he is become the bondslave of his creature, so much so that when the occasion arrives, three-fourths of the act is already done, by the force of the habit alone, before his will is awakened, or drowsily moves in its sleep. The only way for the will to free itself here is not to wait for the occasion to come, but be astir betimes, keep the occasion at arm's length, and register many a determination and firm protest and fervent prayer against the habit. He who neglects to do this in the interval has himself to blame for being overcome every time that he falls upon the occasion which brings into play the evil habit.
Readings.—St. Thos., 1a 2æ, q. 49, art. 4, ad 1, 2; ib., q. 50, art. 3, in corp., ad. 1, 2; ib., q. 51, art. 1, in corp.; ib., q. 53, art. 3, in corp.; Ar., Eth., II., i.; ib., III., v., 10-14; ib., II., iv., 1, 2, 4.