Objection 1. It would seem that contrition is not "an assumed sorrow for sins, together with the purpose of confessing them and of making satisfaction for them," as some define it. For, as Augustine states (De Civ. Dei xiv, 6), "sorrow is for those things that happen against our will." But this does not apply to sin. Therefore contrition is not sorrow for sins.
Objection 2. Further, contrition is given us by God. But what is given is not assumed. Therefore contrition is not an assumed sorrow.
Objection 3. Further, satisfaction and confession are necessary for the remission of the punishment which was not remitted by contrition. But sometimes the whole punishment is remitted in contrition. Therefore it is not always necessary for the contrite person to have the purpose of confessing and of making satisfaction.
On the contrary, stands the definition.
I answer that, As stated in Sirach 10:15, "pride is the beginning of all sin," because thereby man clings to his own judgment, and strays from the Divine commandments. Consequently that which destroys sin must needs make man give up his own judgment. Now he that persists in his own judgment, is called metaphorically rigid and hard: wherefore anyone is said to be broken when he is torn from his own judgment. But, in material things, whence these expressions are transferred to spiritual things, there is a difference between breaking and crushing or contrition, as stated in Meteor. iv, in that we speak of breaking when a thing is sundered into large parts, but of crushing or contrition when that which was in itself solid is reduced to minute particles. And since, for the remission of sin, it is necessary that man should put aside entirely his attachment to sin, which implies a certain state of continuity and solidity in his mind, therefore it is that the act through which sin is cast aside is called contrition metaphorically.
In this contrition several things are to be observed, viz. the very substance of the act, the way of acting, its origin and its effect: in respect of which we find that contrition has been defined in various ways. For, as regards the substance of the act, we have the definition given above: and since the act of contrition is both an act of virtue, and a part of the sacrament of Penance, its nature as an act of virtue is explained in this definition by mentioning its genus, viz. "sorrow," its object by the words "for sins," and the act of choice which is necessary for an act of virtue, by the word "assumed": while, as a part of the sacrament, it is made manifest by pointing out its relation to the other parts, in the words "together with the purpose of confessing and of making satisfaction."
There is another definition which defines contrition, only as an act of virtue; but at the same time including the difference which confines it to a special virtue, viz. penance, for it is thus expressed: "Contrition is voluntary sorrow for sin whereby man punishes in himself that which he grieves to have done," because the addition of the word "punishes" defines the definition to a special virtue. Another definition is given by Isidore (De Sum. Bono ii, 12) as follows: "Contrition is a tearful sorrow and humility of mind, arising from remembrance of sin and fear of the Judgment." Here we have an allusion to the derivation of the word, when it is said that it is "humility of the mind," because just as pride makes the mind rigid, so is a man humbled, when contrition leads him to give up his mind. Also the external manner is indicated by the word "tearful," and the origin of contrition, by the words, "arising from remembrance of sin," etc. Another definition is taken from the words of Augustine [implicitly on Psalm 46], and indicates the effect of contrition. It runs thus: "Contrition is the sorrow which takes away sin." Yet another is gathered from the words of Gregory (Moral. xxxiii, 11) as follows: "Contrition is humility of the soul, crushing sin between hope and fear." Here the derivation is indicated by saying that contrition is "humility of the soul"; the effect, by the words, "crushing sin"; and the origin, by the words, "between hope and fear." Indeed, it includes not only the principal cause, which is fear, but also its joint cause, which is hope, without which, fear might lead to despair.
Reply to Objection 1. Although sins, when committed, were voluntary, yet when we are contrite for them, they are no longer voluntary, so that they occur against our will; not indeed in respect of the will that we had when we consented to them, but in respect of that which we have now, so as to wish they had never been.
Reply to Objection 2. Contrition is from God alone as to the form that quickens it, but as to the substance of the act, it is from the free-will and from God, Who operates in all works both of nature and of will.
Reply to Objection 3. Although the entire punishment may be remitted by contrition, yet confession and satisfaction are still necessary, both because man cannot be sure that his contrition was sufficient to take away all, and because confession and satisfaction are a matter of precept: wherefore he becomes a transgressor, who confesses not and makes not satisfaction.
Source: Summa Theologica (New Advent)