Summa Theologiae Moralis, vol. I: De Principiis

S. Th. I, q. 83, a. 1, ad 5; I-II, q. 78, a. 2 et 4.

95. A habit is a firm and permanent disposition acquired by the repetition of acts, whereby one readily and promptly breaks forth into similar acts, and it can be defined as: a facility and promptness acquired by repeated acts for performing similar acts. Custom is the very repetition of acts, or the usual manner of acting; whence properly it is distinguished from an acquired habit and is related to it as cause to effect; indeed, also as effect to cause: for custom is the cause of acquiring and strengthening a habit; but with a habit already acquired, it is itself the cause why the custom is continued. In the present question, however, moralists more often use one interchangeably for the other.

Custom can be contracted inadvertently and therefore be undeliberate and involuntary; or it can be contracted knowingly and freely and therefore be voluntary and deliberate. Moreover, once the habit has been contracted, after the subject of it becomes aware, either he effectively retracts it, i.e., with the firm purpose of employing fitting remedies for eradicating it, in which case the habit is actually involuntary, although perhaps it was antecedently voluntarily and freely acquired: — or he freely accepts and preserves it, or does not retract it but neglects to eradicate it, in which case it is actually voluntary and free, although perhaps it was antecedently involuntary or undeliberate.

96. Principles. I. An undeliberate and involuntary habit flows in the same manner as antecedent concupiscence; hence it increases the voluntary as regards inclination, but diminishes it as regards the free.

Noted: 1) if someone solely from such a habit and therefore performs the act entirely inadvertently, this will be non-voluntary and in no way deliberate, and hence neither good nor evil; 2) a deliberate habit, or one freely admitted, if it is effectively retracted, i.e. not by mere displeasure but by a firm resolve to work towards its eradication, becomes actually involuntary; whence the acts following solely from it are involuntary and not imputable, just as the cause itself or the habit; 3) if the habitual person afterwards neglects to apply effective remedies in due time, the habit (from that negligence) again becomes indirectly voluntary and free, and hence the acts following from it will be voluntary and free and imputable.

II. Deliberate and voluntary habit 1) always increases the voluntary as regards inclination; — 2) if, which is rarer, it removes all actual attention and deliberation in subsequent acts, it diminishes the free and consequently the goodness or sin, because it removes the voluntary and free in itself and only leaves the voluntary and free in its cause, if the habit was deliberately acquired, or the indirect voluntary and free, if it was indeliberately acquired, but freely neglected to be removed; — 3) if, however, which is more frequent, it leaves sufficient attention here and now, it does not remove the free in itself but diminishes it to some extent, because ordinarily it clouds the intellect and its actual deliberation, and diminishes indifference; yet this diminution of freedom is perfectly compensated in its cause, because man freely contracted the habit and preserves it, and thus freely wills that very diminution of deliberation and freedom, which shows the will choosing intensely and confirmed in good or in evil. Hence simply the voluntary and the goodness or sin are increased. Whence St. Thomas says that he who sins from habit, i.e., not from inadvertence or passion, but by sole free will uses his habit, sins from sheer malice. Whence two principles:

1. What is done advertently from an involuntary habit, or inadvertently from a voluntary habit, is in some respect more voluntary, but less free.

2. What is done advertently from voluntary habit is not less free, but simply more voluntary.

3. Observation. — A good habit diminishes freedom toward evil; this, however, is an imperfection of the will; whence a good habit perfects the will more.