Objection 1. It would seem that justice is not a virtue. For it is written (Luke 17:10): "When you shall have done all these things that are commanded you, say: We are unprofitable servants; we have done that which we ought to do." Now it is not unprofitable to do a virtuous deed: for Ambrose says (De Officiis ii, 6): "We look to a profit that is estimated not by pecuniary gain but by the acquisition of godliness." Therefore to do what one ought to do, is not a virtuous deed. And yet it is an act of justice. Therefore justice is not a virtue.
Objection 2. Further, that which is done of necessity, is not meritorious. But to render to a man what belongs to him, as justice requires, is of necessity. Therefore it is not meritorious. Yet it is by virtuous actions that we gain merit. Therefore justice is not a virtue.
Objection 3. Further, every moral virtue is about matters of action. Now those things which are wrought externally are not things concerning behavior but concerning handicraft, according to the Philosopher (Metaph. ix) [Didot ed., viii, 8]. Therefore since it belongs to justice to produce externally a deed that is just in itself, it seems that justice is not a moral virtue.
On the contrary, Gregory says (Moral. ii, 49) that "the entire structure of good works is built on four virtues," viz. temperance, prudence, fortitude and justice
I answer that, A human virtue is one "which renders a human act and man himself good" [Ethic. ii, 6, and this can be applied to justice. For a man's act is made good through attaining the rule of reason, which is the rule whereby human acts are regulated. Hence, since justice regulates human operations, it is evident that it renders man's operations good, and, as Tully declares (De Officiis i, 7), good men are so called chiefly from their justice, wherefore, as he says again (De Officiis i, 7) "the luster of virtue appears above all in justice."
Reply to Objection 1. When a man does what he ought, he brings no gain to the person to whom he does what he ought, but only abstains from doing him a harm. He does however profit himself, in so far as he does what he ought, spontaneously and readily, and this is to act virtuously. Hence it is written (Wisdom 8:7) that Divine wisdom "teacheth temperance, and prudence, and justice, and fortitude, which are such things as men (i.e. virtuous men) can have nothing more profitable in life."
Reply to Objection 2. Necessity is twofold. One arises from "constraint," and this removes merit, since it runs counter to the will. The other arises from the obligation of a "command," or from the necessity of obtaining an end, when, to wit, a man is unable to achieve the end of virtue without doing some particular thing. The latter necessity does not remove merit, when a man does voluntarily that which is necessary in this way. It does however exclude the credit of supererogation, according to 1 Corinthians 9:16, "If I preach the Gospel, it is no glory to me, for a necessity lieth upon me."
Reply to Objection 3. Justice is concerned about external things, not by making them, which pertains to art, but by using them in our dealings with other men.
Source: Summa Theologica (New Advent)