I answer that, The subject of a virtue is the power whose act that virtue aims at rectifying. Now justice does not aim at directing an act of the cognitive power, for we are not said to be just through knowing something aright. Hence the subject of justice is not the intellect or reason which is a cognitive power. But since we are said to be just through doing something aright, and because the proximate principle of action is the appetitive power, justice must needs be in some appetitive power as its subject.
Now the appetite is twofold; namely, the will which is in the reason and the sensitive appetite which follows on sensitive apprehension, and is divided into the irascible and the concupiscible, as stated in the First Part (Q. 81, A. 2). Again the act of rendering his due to each man cannot proceed from the sensitive appetite, because sensitive apprehension does not go so far as to be able to consider the relation of one thing to another; but this is proper to the reason. Therefore justice cannot be in the irascible or concupiscible as its subject, but only in the will: hence the Philosopher (Ethic. v, 1) defines justice by an act of the will, as may be seen above (A. 1).
Reply Obj. 1: Since the will is the rational appetite, when the rectitude of the reason which is called truth is imprinted on the will on account of its nighness to the reason, this imprint retains the name of truth; and hence it is that justice sometimes goes by the name of truth.
Reply Obj. 2: The will is borne towards its object consequently on the apprehension of reason: wherefore, since the reason directs one thing in relation to another, the will can will one thing in relation to another, and this belongs to justice.
Reply Obj. 3: Not only the irascible and concupiscible parts are rational by participation, but the entire appetitive faculty, as stated in Ethic. i, 13, because all appetite is subject to reason. Now the will is contained in the appetitive faculty, wherefore it can be the subject of moral virtue. _______________________
FIFTH ARTICLE [II-II, Q. 58, Art. 5]
Whether Justice Is a General Virtue?
Objection 1: It would seem that justice is not a general virtue. For justice is specified with the other virtues, according to Wis. 8:7, "She teacheth temperance and prudence, and justice, and fortitude." Now the "general" is not specified or reckoned together with the species contained under the same "general." Therefore justice is not a general virtue.
Obj. 2: Further, as justice is accounted a cardinal virtue, so are temperance and fortitude. Now neither temperance nor fortitude is reckoned to be a general virtue. Therefore neither should justice in any way be reckoned a general virtue.
Obj. 3: Further, justice is always towards others, as stated above (A. 2). But a sin committed against one's neighbor cannot be a general sin, because it is condivided with sin committed against oneself. Therefore neither is justice a general virtue.