Objection 1. It would seem that the virtue of penance is not a species of justice. For justice is not a theological but a moral virtue, as was shown in the II-II, 62, 3. But penance seems to be a theological virtue, since God is its object, for it makes satisfaction to God, to Whom, moreover, it reconciles the sinner. Therefore it seems that penance is not a species of justice.
Objection 2. Further, since justice is a moral virtue it observes the mean. Now penance does not observe the mean, but rather goes to the extreme, according to Jeremiah 6:26: "Make thee mourning as for an only son, a bitter lamentation." Therefore penance is not a species of justice.
Objection 3. Further, there are two species of justice, as stated in Ethic. v, 4, viz. "distributive" and "commutative." But penance does not seem to be contained under either of them. Therefore it seems that penance is not a species of justice.
Objection 4. Further, a gloss on Luke 6:21, "Blessed are ye that weep now," says: "It is prudence that teaches us the unhappiness of earthly things and the happiness of heavenly things." But weeping is an act of penance. Therefore penance is a species of prudence rather than of justice.
On the contrary, Augustine says in De Poenitentia [De vera et falsa Poenitentia, the authorship of which is unknown]: "Penance is the vengeance of the sorrowful, ever punishing in them what they are sorry for having done." But to take vengeance is an act of justice, wherefore Tully says (De Inv. Rhet. ii) that one kind of justice is called vindictive. Therefore it seems that penance is a species of justice.
I answer that, As stated above (1, ad 2) penance is a special virtue not merely because it sorrows for evil done (since charity would suffice for that), but also because the penitent grieves for the sin he has committed, inasmuch as it is an offense against God, and purposes to amend. Now amendment for an offense committed against anyone is not made by merely ceasing to offend, but it is necessary to make some kind of compensation, which obtains in offenses committed against another, just as retribution does, only that compensation is on the part of the offender, as when he makes satisfaction, whereas retribution is on the part of the person offended against. Each of these belongs to the matter of justice, because each is a kind of commutation. Wherefore it is evident that penance, as a virtue, is a part of justice.
It must be observed, however, that according to the Philosopher (Ethic. v, 6) a thing is said to be just in two ways, simply and relatively. A thing is just simply when it is between equals, since justice is a kind of equality, and he calls this the politic or civil just, because all citizens are equal, in the point of being immediately under the ruler, retaining their freedom. But a thing is just relatively when it is between parties of whom one is subject to the other, as a servant under his master, a son under his father, a wife under her husband. It is this kind of just that we consider in penance. Wherefore the penitent has recourse to God with a purpose of amendment, as a servant to his master, according to Psalm 122:2: "Behold, as the eyes of servants are on the hands of their masters... so are our eyes unto the Lord our God, until He have mercy on us"; and as a son to his father, according to Luke 15:21: "Father, I have sinned against heaven and before thee"; and as a wife to her husband, according to Jeremiah 3:1: "Thou hast prostituted thyself to many lovers; nevertheless return to Me, saith the Lord."
Reply to Objection 1. As stated in Ethic. v, 1, justice is a virtue towards another person, and the matter of justice is not so much the person to whom justice is due as the thing which is the subject of distribution or commutation. Hence the matter of penance is not God, but human acts, whereby God is offended or appeased; whereas God is as one to whom justice is due. Wherefore it is evident that penance is not a theological virtue, because God is not its matter or object.
Reply to Objection 2. The mean of justice is the equality that is established between those between whom justice is, as stated in Ethic. v. But in certain cases perfect equality cannot be established, on account of the excellence of one, as between father and son, God and man, as the Philosopher states (Ethic. viii, 14), wherefore in such cases, he that falls short of the other must do whatever he can. Yet this will not be sufficient simply, but only according to the acceptance of the higher one; and this is what is meant by ascribing excess to penance.
Reply to Objection 3. As there is a kind of commutation in favors, when, to wit, a man gives thanks for a favor received, so also is there commutation in the matter of offenses, when, on account of an offense committed against another, a man is either punished against his will, which pertains to vindictive justice, or makes amends of his own accord, which belongs to penance, which regards the person of the sinner, just as vindictive justice regards the person of the judge. Therefore it is evident that both are comprised under commutative justice.
Reply to Objection 4. Although penance is directly a species of justice, yet, in a fashion, it comprises things pertaining to all the virtues; for inasmuch as there is a justice of man towards God, it must have a share in matter pertaining to the theological virtues, the object of which is God. Consequently penance comprises faith in Christ's Passion, whereby we are cleansed of our sins, hope for pardon, and hatred of vice, which pertains to charity. Inasmuch as it is a moral virtue, it has a share of prudence, which directs all the moral virtues: but from the very nature of justice, it has not only something belonging to justice, but also something belonging to temperance and fortitude, inasmuch as those things which cause pleasure, and which pertain to temperance, and those which cause terror, which fortitude moderates, are objects of commutative justice. Accordingly it belongs to justice both to abstain from pleasure, which belongs to temperance, and to bear with hardships, which belongs to fortitude.
Source: Summa Theologica (New Advent)