Objection 1. It would seem that lawyers have unfittingly defined justice as being "the perpetual and constant will to render to each one his right" [Digest. i, 1; De Just. et Jure 10. For, according to the Philosopher (Ethic. v, 1), justice is a habit which makes a man "capable of doing what is just, and of being just in action and in intention." Now "will" denotes a power, or also an act. Therefore justice is unfittingly defined as being a will.
Objection 2. Further, rectitude of the will is not the will; else if the will were its own rectitude, it would follow that no will is unrighteous. Yet, according to Anselm (De Veritate xii), justice is rectitude. Therefore justice is not the will.
Objection 3. Further, no will is perpetual save God's. If therefore justice is a perpetual will, in God alone will there be justice.
Objection 4. Further, whatever is perpetual is constant, since it is unchangeable. Therefore it is needless in defining justice, to say that it is both "perpetual" and "constant."
Objection 5. Further, it belongs to the sovereign to give each one his right. Therefore, if justice gives each one his right, it follows that it is in none but the sovereign: which is absurd.
Objection 6. Further, Augustine says (De Moribus Eccl. xv) that "justice is love serving God alone." Therefore it does not render to each one his right.
I answer that, The aforesaid definition of justice is fitting if understood aright. For since every virtue is a habit that is the principle of a good act, a virtue must needs be defined by means of the good act bearing on the matter proper to that virtue. Now the proper matter of justice consists of those things that belong to our intercourse with other men, as shall be shown further on (2). Hence the act of justice in relation to its proper matter and object is indicated in the words, "Rendering to each one his right," since, as Isidore says (Etym. x), "a man is said to be just because he respects the rights [jus] of others."
Now in order that an act bearing upon any matter whatever be virtuous, it requires to be voluntary, stable, and firm, because the Philosopher says (Ethic. ii, 4) that in order for an act to be virtuous it needs first of all to be done "knowingly," secondly to be done "by choice," and "for a due end," thirdly to be done "immovably." Now the first of these is included in the second, since "what is done through ignorance is involuntary" (Ethic. iii, 1). Hence the definition of justice mentions first the "will," in order to show that the act of justice must be voluntary; and mention is made afterwards of its "constancy" and "perpetuity" in order to indicate the firmness of the act.
Accordingly, this is a complete definition of justice; save that the act is mentioned instead of the habit, which takes its species from that act, because habit implies relation to act. And if anyone would reduce it to the proper form of a definition, he might say that "justice is a habit whereby a man renders to each one his due by a constant and perpetual will": and this is about the same definition as that given by the Philosopher (Ethic. v, 5) who says that "justice is a habit whereby a man is said to be capable of doing just actions in accordance with his choice."
Reply to Objection 1. Will here denotes the act, not the power: and it is customary among writers to define habits by their acts: thus Augustine says (Tract. in Joan. xl) that "faith is to believe what one sees not."
Reply to Objection 2. Justice is the same as rectitude, not essentially but causally; for it is a habit which rectifies the deed and the will.
Reply to Objection 3. The will may be called perpetual in two ways. First on the part of the will's act which endures for ever, and thus God's will alone is perpetual. Secondly on the part of the subject, because, to wit, a man wills to do a certain thing always. and this is a necessary condition of justice. For it does not satisfy the conditions of justice that one wish to observe justice in some particular matter for the time being, because one could scarcely find a man willing to act unjustly in every case; and it is requisite that one should have the will to observe justice at all times and in all cases.
Reply to Objection 4. Since "perpetual" does not imply perpetuity of the act of the will, it is not superfluous to add "constant": for while the "perpetual will" denotes the purpose of observing justice always, "constant" signifies a firm perseverance in this purpose.
Reply to Objection 5. A judge renders to each one what belongs to him, by way of command and direction, because a judge is the "personification of justice," and "the sovereign is its guardian" (Ethic. v, 4). On the other hand, the subjects render to each one what belongs to him, by way of execution.
Reply to Objection 6. Just as love of God includes love of our neighbor, as stated above (Question 25, Article 1), so too the service of God includes rendering to each one his due.
Source: Summa Theologica (New Advent)