(b) Justice, on the contrary, regulates external operations for the benefit, not of the subject, but of other persons whom they affect, in order that the subject in dealing with others may avoid inequality, which means excess on one side and defect on the other side. Hence, the middle way of justice is discovered by reason from a consideration of external things or acts owed to other persons (the mean of reason and of the thing), and so it does not vary with the circumstances of the subject. If the real value of a horse is $100—it makes no difference whether the seller be a prince or a peasant, whether the buyer be rich or poor—the just payment will be $100. Excess will be unfair to the buyer, deficiency to the seller.
1712. Though the mean of justice is determined, not by reference to the person who acts, but by reference to some external thing, it may be that this external thing cannot be evaluated without consideration of the person to whom justice is owed.
(a) In distributive justice this is always the case, for the mean of the thing in distributions consists in equality between relative proportions of distributions and relative merits or abilities of persons to whom distributions are made. Hence, distributive justice must consider the conditions of the person to whom it is owed as compared with the conditions of other persons, in order to observe equality by giving proper shares to all.
(b) In commutative justice, this is sometimes the case, namely, when the condition of a person who has been offended (e.g., that he is a ruler) increases the debt of satisfaction that is owed him; for the mean of the thing in commutative justice is equality between the payment and the debt.
1713. Is observance of the mean of the thing sufficient to make an act just, no matter what may be the dispositions of the subject?
(a) If there is question of material justice, the reply is in the affirmative, for a virtue is said to be exercised materially when its mean is observed. The mean of fortitude and temperance cannot be observed without reference to the condition of the subject (e.g., he is not brave who undertakes a difficult task that is beyond his strength); but the same is not true of justice (e.g., he is just who pays the last penny of a debt though the payment was beyond his means and required a sacrifice).
(b) If there is question of formal justice, the reply is in the negative, for a virtue is said to be exercised formally (i.e., from a virtuous habit) when the motive of the subject and the circumstances are agreeable to reason. Thus, he who performs deeds of valor purely out of vainglory exercises fortitude materially, not formally; and likewise he who pays his debts faithfully, merely in order to avoid the penalties of the law, exercises justice materially but not formally.
1714. Comparison of Justice and the Other Virtues.—The differences between particular justice and the other moral virtues are, therefore, the following:
(a) justice is for the good of another, the other virtues for the good of the agent himself;
(b) justice deals with external actions and things, the others with the passions;