Summa summae S. Thomae, Tomus III

Justice is defined in law, from Ulpian : « The constant and perpetual will to render to each his right. »

By will is not understood the very power of willing, but the act¹ ; whence a habit is defined through the act : which is not unusual. It is called « perpetual », not on the part of the act, which endures perpetually, but on the part of the object, insofar as the just man wills to render to everyone his right perpetually and in all things : whence, if anyone wishes to reduce this definition to due form, let him say that justice is « the virtue inclining the will constantly to render forever to everyone his right ». Hence you have that it belongs to the reason of justice properly so called that it be toward another.

I say 2°: The remote material object of justice are external things which are commuted or distributed. The proximate material object are the actions by which these things are commuted and distributed. Because justice differs in this from the other moral virtues, that these rectify primarily and per se man in himself, but justice in order to another². Now the ordination of one man to another is not attended to according to interior affections or passions, but according to exterior actions and things by which men can communicate with one another, such as, as to things, goods of fortune, money, houses, fields, grains, etc. ; as to actions, those by which these things are commuted, namely sale, payment, loan for consumption, loan for use, lease, etc. ; likewise also those actions in which no exterior thing intervenes, as honor, praise, detraction, etc. Therefore.

The formal object quod of justice is right taken for the just, or the equal owed to another³ ; because this justice principally intends and effects, as appears from its definition, and thereby it is distinguished from the other virtues. Whence he is called just who makes or preserves this equality, unjust who does not preserve it.

The formal object quo is the ratio of debt, or right in another, taken for the legitimate faculty of possessing, alienating, etc. For the reason why I render the equal to another is because I owe it to him ; I owe it however because that other has a right to it : for debt and right are correlatives.

I say 3°: The subject of justice is the will⁴. For it is not the intellect, because justice is not ordered per se and primarily to direct any cognitive act ; nor is it the sensitive appetite, because the sensitive appetite follows sensitive apprehension, but sensitive apprehension does not consider the proportion of one thing to another, which pertains to justice ; for this is proper to reason. Therefore.