Objection 1. It would seem that the act of justice is not to render to each one his own. For Augustine (De Trin. xiv, 9) ascribes to justice the act of succoring the needy. Now in succoring the needy we give them what is not theirs but ours. Therefore the act of justice does not consist in rendering to each one his own.
Objection 2. Further, Tully says (De Offic. i, 7) that "beneficence which we may call kindness or liberality, belongs to justice." Now it pertains to liberality to give to another of one's own, not of what is his. Therefore the act of justice does not consist in rendering to each one his own.
Objection 3. Further, it belongs to justice not only to distribute things duly, but also to repress injurious actions, such as murder, adultery and so forth. But the rendering to each one of what is his seems to belong solely to the distribution of things. Therefore the act of justice is not sufficiently described by saying that it consists in rendering to each one his own.
On the contrary, Ambrose says (De Offic. i, 24): "It is justice that renders to each one what is his, and claims not another's property; it disregards its own profit in order to preserve the common equity."
I answer that, As stated above (A8,10), the matter of justice is an external operation in so far as either it or the thing we use by it is made proportionate to some other person to whom we are related by justice. Now each man's own is that which is due to him according to equality of proportion. Therefore the proper act of justice is nothing else than to render to each one his own.
Reply to Objection 1. Since justice is a cardinal virtue, other secondary virtues, such as mercy, liberality and the like are connected with it, as we shall state further on (80, 1). Wherefore to succor the needy, which belongs to mercy or pity, and to be liberally beneficent, which pertains to liberality, are by a kind of reduction ascribed to justice as to their principal virtue.
This suffices for the Reply to the Second Objection.
Reply to Objection 3. As the Philosopher states (Ethic. v, 4), in matters of justice, the name of "profit" is extended to whatever is excessive, and whatever is deficient is called "loss." The reason for this is that justice is first of all and more commonly exercised in voluntary interchanges of things, such as buying and selling, wherein those expressions are properly employed; and yet they are transferred to all other matters of justice. The same applies to the rendering to each one of what is his own.
Source: Summa Theologica (New Advent)