Summa Th. II-II EN Qu.60


In due sequence we must consider judgment, under which head there are six points of inquiry:

(1) Whether judgment is an act of justice?

(2) Whether it is lawful to judge?

(3) Whether judgment should be based on suspicions?

(4) Whether doubts should be interpreted favorably?

(5) Whether judgment should always be given according to the written law?

(6) Whether judgment is perverted by being usurped?

Whether judgment is an act of justice?

Objection: 1. It would seem that judgment is not an act of justice. The Philosopher says (Ethic. i, 3) that "everyone judges well of what he knows," so that judgment would seem to belong to the cognitive faculty. Now the cognitive faculty is perfected by prudence. Therefore judgment belongs to prudence rather than to justice, which is in the will, as stated above (Question [58], Article [4]).
2. Further, the Apostle says (1Co 2,15): "The spiritual man judgeth all things." Now man is made spiritual chiefly by the virtue of charity, which "is poured forth in our hearts by the Holy Ghost Who is given to us" (Rm 5,5). Therefore judgment belongs to charity rather than to justice.
3. Further, it belongs to every virtue to judge aright of its proper matter, because "the virtuous man is the rule and measure in everything," according to the Philosopher (Ethic. iii, 4). Therefore judgment does not belong to justice any more than to the other moral virtues.
4. Further, judgment would seem to belong only to judges. But the act of justice is to be found in every just man. Since then judges are not the only just men, it seems that judgment is not the proper act of justice.

On the contrary It is written (Ps 93,15): "Until justice be turned into judgment."
I answer that Judgment properly denotes the act of a judge as such. Now a judge [judex] is so called because he asserts the right [jus dicens] and right is the object of justice, as stated above (Question [57], Article [1]). Consequently the original meaning of the word "judgment" is a statement or decision of the just or right. Now to decide rightly about virtuous deeds proceeds, properly speaking, from the virtuous habit; thus a chaste person decides rightly about matters relating to chastity. Therefore judgment, which denotes a right decision about what is just, belongs properly to justice. For this reason the Philosopher says (Ethic. v, 4) that "men have recourse to a judge as to one who is the personification of justice."

Reply to Objection: 1. The word "judgment," from its original meaning of a right decision about what is just, has been extended to signify a right decision in any matter whether speculative or practical. Now a right judgment in any matter requires two things. The first is the virtue itself that pronounces judgment: and in this way, judgment is an act of reason, because it belongs to the reason to pronounce or define. The other is the disposition of the one who judges, on which depends his aptness for judging aright. In this way, in matters of justice, judgment proceeds from justice, even as in matters of fortitude, it proceeds from fortitude. Accordingly judgment is an act of justice in so far as justice inclines one to judge aright, and of prudence in so far as prudence pronounces judgment: wherefore (synesis) (judging well according to common law) which belongs to prudence is said to "judge rightly," as stated above (Question [51], Article [3]).
2. The spiritual man, by reason of the habit of charity, has an inclination to judge aright of all things according to the Divine rules; and it is in conformity with these that he pronounces judgment through the gift of wisdom: even as the just man pronounces judgment through the virtue of prudence conformably with the ruling of the law.
3. The other virtues regulate man in himself, whereas justice regulates man in his dealings with others, as shown above (Question [58], Article [2]). Now man is master in things concerning himself, but not in matters relating to others. Consequently where the other virtues are in question, there is no need for judgment other than that of a virtuous man, taking judgment in its broader sense, as explained above (ad 1). But in matters of justice, there is further need for the judgment of a superior, who is "able to reprove both, and to put his hand between both" [*Jb 9,33]. Hence judgment belongs more specifically to justice than to any other virtue.
4. Justice is in the sovereign as a master-virtue [*Cf. Question [58], Article [6]], commanding and prescribing what is just; while it is in the subjects as an executive and administrative virtue. Hence judgment, which denotes a decision of what is just, belongs to justice, considered as existing chiefly in one who has authority.

Whether it is lawful to judge?

Objection: 1. It would seem unlawful to judge. For nothing is punished except what is unlawful. Now those who judge are threatened with punishment, which those who judge not will escape, according to Mt 7,1, "Judge not, and ye shall not be judged." Therefore it is unlawful to judge.
2. Further, it is written (Rm 14,4): "Who art thou that judgest another man's servant. To his own lord he standeth or falleth." Now God is the Lord of all. Therefore to no man is it lawful to judge.
3. Further, no man is sinless, according to 1Jn 1,8, "If we say that we have no sin, we deceive ourselves." Now it is unlawful for a sinner to judge, according to Rm 2,1, "Thou art inexcusable, O man, whosoever thou art, that judgest; for wherein thou judgest another, thou condemnest thyself, for thou dost the same things which thou judgest." Therefore to no man is it lawful to judge.

On the contrary It is written (Dt 16,18): "Thou shalt appoint judges and magistrates in all thy gates . . . that they may judge the people with just judgment."
I answer that Judgment is lawful in so far as it is an act of justice. Now it follows from what has been stated above (Article [1], ad 1,3) that three conditions are requisite for a judgment to be an act of justice: first, that it proceed from the inclination of justice; secondly, that it come from one who is in authority; thirdly, that it be pronounced according to the right ruling of prudence. If any one of these be lacking, the judgment will be faulty and unlawful. First, when it is contrary to the rectitude of justice, and then it is called "perverted" or "unjust": secondly, when a man judges about matters wherein he has no authority, and this is called judgment "by usurpation": thirdly, when the reason lacks certainty, as when a man, without any solid motive, forms a judgment on some doubtful or hidden matter, and then it is called judgment by "suspicion" or "rash" judgment.

Reply to Objection: 1. In these words our Lord forbids rash judgment which is about the inward intention, or other uncertain things, as Augustine states (De Serm. Dom. in Monte ii, 18). Or else He forbids judgment about Divine things, which we ought not to judge, but simply believe, since they are above us, as Hilary declares in his commentary on Mt 5. Or again according to Chrysostom [*Hom. xvii in Matth. in the Opus Imperfectum falsely ascribed to St. John of the Cross], He forbids the judgment which proceeds not from benevolence but from bitterness of heart.
2. A judge is appointed as God's servant; wherefore it is written (Dt 1,16): "Judge that which is just," and further on (Dt 1,17), "because it is the judgment of God."
3. Those who stand guilty of grievous sins should not judge those who are guilty of the same or lesser sins, as Chrysostom [*Hom. xxiv] says on the words of Mt 7,1, "Judge not." Above all does this hold when such sins are public, because there would be an occasion of scandal arising in the hearts of others. If however they are not public but hidden, and there be an urgent necessity for the judge to pronounce judgment, because it is his duty, he can reprove or judge with humility and fear. Hence Augustine says (De Serm. Dom. in Monte ii, 19): "If we find that we are guilty of the same sin as another man, we should groan together with him, and invite him to strive against it together with us." And yet it is not through acting thus that a man condemns himself so as to deserve to be condemned once again, but when, in condemning another, he shows himself to be equally deserving of condemnation on account of another or a like sin.

Whether it is unlawful to form a judgment from suspicions?

Objection: 1. It would seem that it is not unlawful to form a judgment from suspicions. For suspicion is seemingly an uncertain opinion about an evil, wherefore the Philosopher states (Ethic. vi, 3) that suspicion is about both the true and the false. Now it is impossible to have any but an uncertain opinion about contingent singulars. Since then human judgment is about human acts, which are about singular and contingent matters, it seems that no judgment would be lawful, if it were not lawful to judge from suspicions.
2. Further, a man does his neighbor an injury by judging him unlawfully. But an evil suspicion consists in nothing more than a man's opinion, and consequently does not seem to pertain to the injury of another man. Therefore judgment based on suspicion is not unlawful.
3. Further, if it is unlawful, it must needs be reducible to an injustice, since judgment is an act of justice, as stated above (Article [1]). Now an injustice is always a mortal sin according to its genus, as stated above (Question [59], Article [4]). Therefore a judgment based on suspicion would always be a mortal sin, if it were unlawful. But this is false, because "we cannot avoid suspicions," according to a gloss of Augustine (Tract. xc in Joan.) on 1Co 4,5, "Judge not before the time." Therefore a judgment based on suspicion would seem not to be unlawful.

On the contrary Chrysostom [*Hom. xvii in Matth. in the Opus Imperfectum falsely ascribed to St. John of the Cross] in comment on the words of Mt 7,1, "Judge not," etc., says: "By this commandment our Lord does not forbid Christians to reprove others from kindly motives, but that Christian should despise Christian by boasting his own righteousness, by hating and condemning others for the most part on mere suspicion."
I answer that As Tully says (De Invent. Rhet. ii), suspicion denotes evil thinking based on slight indications, and this is due to three causes. First, from a man being evil in himself, and from this very fact, as though conscious of his own wickedness, he is prone to think evil of others, according to Qo 10,3, "The fool when he walketh in the way, whereas he himself is a fool, esteemeth all men fools." Secondly, this is due to a man being ill-disposed towards another: for when a man hates or despises another, or is angry with or envious of him, he is led by slight indications to think evil of him, because everyone easily believes what he desires. Thirdly, this is due to long experience: wherefore the Philosopher says (Rhet. ii, 13) that "old people are very suspicious, for they have often experienced the faults of others." The first two causes of suspicion evidently connote perversity of the affections, while the third diminishes the nature of suspicion, in as much as experience leads to certainty which is contrary to the nature of suspicion. Consequently suspicion denotes a certain amount of vice, and the further it goes, the more vicious it is.Now there are three degrees of suspicion. The first degree is when a man begins to doubt of another's goodness from slight indications. This is a venial and a light sin; for "it belongs to human temptation without which no man can go through this life," according to a gloss on 1Co 4,5, "Judge not before the time." The second degree is when a man, from slight indications, esteems another man's wickedness as certain. This is a mortal sin, if it be about a grave matter, since it cannot be without contempt of one's neighbor. Hence the same gloss goes on to say: "If then we cannot avoid suspicions, because we are human, we must nevertheless restrain our judgment, and refrain from forming a definite and fixed opinion." The third degree is when a judge goes so far as to condemn a man on suspicion: this pertains directly to injustice, and consequently is a mortal sin.

Reply to Objection: 1. Some kind of certainty is found in human acts, not indeed the certainty of a demonstration, but such as is befitting the matter in point, for instance when a thing is proved by suitable witnesses.
2. From the very fact that a man thinks evil of another without sufficient cause, he despises him unduly, and therefore does him an injury.
3. Since justice and injustice are about external operations, as stated above (Question [58], Articles [8],10,11; Question [59], Article [1], ad 3), the judgment of suspicion pertains directly to injustice when it is betrayed by external action, and then it is a mortal sin, as stated above. The internal judgment pertains to justice, in so far as it is related to the external judgment, even as the internal to the external act, for instance as desire is related to fornication, or anger to murder.

Whether doubts should be interpreted for the best?

Objection: 1. It would seem that doubts should not be interpreted for the best. Because we should judge from what happens for the most part. But it happens for the most part that evil is done, since "the number of fools is infinite" (Qo 1,15), "for the imagination and thought of man's heart are prone to evil from his youth" (Gn 8,21). Therefore doubts should be interpreted for the worst rather than for the best.
2. Further, Augustine says (De Doctr. Christ. i, 27) that "he leads a godly and just life who is sound in his estimate of things, and turns neither to this side nor to that." Now he who interprets a doubtful point for the best, turns to one side. Therefore this should not be done.
3. Further, man should love his neighbor as himself. Now with regard to himself, a man should interpret doubtful matters for the worst, according to Jb 9,28, "I feared all my works." Therefore it seems that doubtful matters affecting one's neighbor should be interpreted for the worst.

On the contrary A gloss on Rm 14,3, "He that eateth not, let him not judge him that eateth," says: "Doubts should be interpreted in the best sense."
I answer that As stated above (Article [3], ad 2), things from the very fact that a man thinks ill of another without sufficient cause, he injures and despises him. Now no man ought to despise or in any way injure another man without urgent cause: and, consequently, unless we have evident indications of a person's wickedness, we ought to deem him good, by interpreting for the best whatever is doubtful about him.

Reply to Objection: 1. He who interprets doubtful matters for the best, may happen to be deceived more often than not; yet it is better to err frequently through thinking well of a wicked man, than to err less frequently through having an evil opinion of a good man, because in the latter case an injury is inflicted, but not in the former.
2. It is one thing to judge of things and another to judge of men. For when we judge of things, there is no question of the good or evil of the thing about which we are judging, since it will take no harm no matter what kind of judgment we form about it; but there is question of the good of the person who judges, if he judge truly, and of his evil if he judge falsely because "the true is the good of the intellect, and the false is its evil," as stated in Ethic. vi, 2, wherefore everyone should strive to make his judgment accord with things as they are. On the other hand when we judge of men, the good and evil in our judgment is considered chiefly on the part of the person about whom judgment is being formed; for he is deemed worthy of honor from the very fact that he is judged to be good, and deserving of contempt if he is judged to be evil. For this reason we ought, in this kind of judgment, to aim at judging a man good, unless there is evident proof of the contrary. And though we may judge falsely, our judgment in thinking well of another pertains to our good feeling and not to the evil of the intellect, even as neither does it pertain to the intellect's perfection to know the truth of contingent singulars in themselves.
3. One may interpret something for the worst or for the best in two ways. First, by a kind of supposition; and thus, when we have to apply a remedy to some evil, whether our own or another's, in order for the remedy to be applied with greater certainty of a cure, it is expedient to take the worst for granted, since if a remedy be efficacious against a worse evil, much more is it efficacious against a lesser evil. Secondly we may interpret something for the best or for the worst, by deciding or determining, and in this case when judging of things we should try to interpret each thing according as it is, and when judging of persons, to interpret things for the best as stated above.

Whether we should always judge according to the written law?

Objection: 1. It would seem that we ought not always to judge according to the written law. For we ought always to avoid judging unjustly. But written laws sometimes contain injustice, according to Is 10,1, "Woe to them that make wicked laws, and when they write, write injustice." Therefore we ought not always to judge according to the written law.
2. Further, judgment has to be formed about individual happenings. But no written law can cover each and every individual happening, as the Philosopher declares (Ethic. v, 10). Therefore it seems that we are not always bound to judge according to the written law.
3. Further, a law is written in order that the lawgiver's intention may be made clear. But it happens sometimes that even if the lawgiver himself were present he would judge otherwise. Therefore we ought not always to judge according to the written law.

On the contrary Augustine says (De Vera Relig. xxxi): "In these earthly laws, though men judge about them when they are making them, when once they are established and passed, the judges may judge no longer of them, but according to them."
I answer that As stated above (Article [1]), judgment is nothing else but a decision or determination of what is just. Now a thing becomes just in two ways: first by the very nature of the case, and this is called "natural right," secondly by some agreement between men, and this is called "positive right," as stated above (Question [57], Article [2]). Now laws are written for the purpose of manifesting both these rights, but in different ways. For the written law does indeed contain natural right, but it does not establish it, for the latter derives its force, not from the law but from nature: whereas the written law both contains positive right, and establishes it by giving it force of authority.Hence it is necessary to judge according to the written law, else judgment would fall short either of the natural or of the positive right.

Reply to Objection: 1. Just as the written law does not give force to the natural right, so neither can it diminish or annul its force, because neither can man's will change nature. Hence if the written law contains anything contrary to the natural right, it is unjust and has no binding force. For positive right has no place except where "it matters not," according to the natural right, "whether a thing be done in one way or in another"; as stated above (Question [57], Article [2], ad 2). Wherefore such documents are to be called, not laws, but rather corruptions of law, as stated above (I-II 95,2): and consequently judgment should not be delivered according to them.
2. Even as unjust laws by their very nature are, either always or for the most part, contrary to the natural right, so too laws that are rightly established, fail in some cases, when if they were observed they would be contrary to the natural right. Wherefore in such cases judgment should be delivered, not according to the letter of the law, but according to equity which the lawgiver has in view. Hence the jurist says [*Digest. i, 3; De leg. senatusque consult. 25]: "By no reason of law, or favor of equity, is it allowable for us to interpret harshly, and render burdensome, those useful measures which have been enacted for the welfare of man." In such cases even the lawgiver himself would decide otherwise; and if he had foreseen the case, he might have provided for it by law.
3. This suffices for the Reply to the Third Objection.

Whether judgment is rendered perverse by being usurped?

Objection: 1. It would seem that judgment is not rendered perverse by being usurped. For justice is rectitude in matters of action. Now truth is not impaired, no matter who tells it, but it may suffer from the person who ought to accept it. Therefore again justice loses nothing, no matter who declares what is just, and this is what is meant by judgment.
2. Further, it belongs to judgment to punish sins. Now it is related to the praise of some that they punished sins without having authority over those whom they punished; such as Moses in slaying the Egyptian (Ex 2,12), and Phinees the son of Eleazar in slaying Zambri the son of Salu (Nb 25,7-14), and "it was reputed to him unto justice" (Ps 105,31). Therefore usurpation of judgment pertains not to injustice.
3. Further, spiritual power is distinct from temporal. Now prelates having spiritual power sometimes interfere in matters concerning the secular power. Therefore usurped judgment is not unlawful.
4. Further, even as the judge requires authority in order to judge aright, so also does he need justice and knowledge, as shown above (Article [1], ad 1,3; Article [2]). But a judgment is not described as unjust, if he who judges lacks the habit of justice or the knowledge of the law. Neither therefore is it always unjust to judge by usurpation, i.e. without authority.

On the contrary It is written (Rm 14,4): "Who art thou that judgest another man's servant?"
I answer that Since judgment should be pronounced according to the written law, as stated above (Article [5]), he that pronounces judgment, interprets, in a way, the letter of the law, by applying it to some particular case. Now since it belongs to the same authority to interpret and to make a law, just as a law cannot be made save by public authority, so neither can a judgment be pronounced except by public authority, which extends over those who are subject to the community. Wherefore even as it would be unjust for one man to force another to observe a law that was not approved by public authority, so too it is unjust, if a man compels another to submit to a judgment that is pronounced by other than the public authority.

Reply to Objection: 1. When the truth is declared there is no obligation to accept it, and each one is free to receive it or not, as he wishes. On the other hand judgment implies an obligation, wherefore it is unjust for anyone to be judged by one who has no public authority.
2. Moses seems to have slain the Egyptian by authority received as it were, by divine inspiration; this seems to follow from Ac 7,24-25, where it is said that "striking the Egyptian . . . he thought that his brethren understood that God by his hand would save Israel [Vulg.: 'them']." Or it may be replied that Moses slew the Egyptian in order to defend the man who was unjustly attacked, without himself exceeding the limits of a blameless defence. Wherefore Ambrose says (De Offic. i, 36) that "whoever does not ward off a blow from a fellow man when he can, is as much in fault as the striker"; and he quotes the example of Moses. Again we may reply with Augustine (Questions. Exod. qu. 2) [*Cf. Contra Faust. xxii, 70] that just as "the soil gives proof of its fertility by producing useless herbs before the useful seeds have grown, so this deed of Moses was sinful although it gave a sign of great fertility," in so far, to wit, as it was a sign of the power whereby he was to deliver his people.With regard to Phinees the reply is that he did this out of zeal for God by Divine inspiration; or because though not as yet high-priest, he was nevertheless the high-priest's son, and this judgment was his concern as of the other judges, to whom this was commanded [*Ex 22,20; Lv 20; Dt 13,17].
3. The secular power is subject to the spiritual, even as the body is subject to the soul. Consequently the judgment is not usurped if the spiritual authority interferes in those temporal matters that are subject to the spiritual authority or which have been committed to the spiritual by the temporal authority.
4. The habits of knowledge and justice are perfections of the individual, and consequently their absence does not make a judgment to be usurped, as in the absence of public authority which gives a judgment its coercive force.


We must now consider the parts of justice; (1) the subjective parts, which are the species of justice, i.e. distributive and commutative justice; (2) the quasi-integral parts; (3) the quasi-potential parts, i.e. the virtues connected with justice. The first consideration will be twofold: (1) The parts of justice; (2) their opposite vices. And since restitution would seem to be an act of commutative justice, we must consider (1) the distinction between commutative and distributive justice; (2) restitution.

Under the first head there are four points of inquiry:

(1) Whether there are two species of justice, viz. distributive and commutative?

(2) Whether in either case the mean is take in the same way?

(3) Whether their matter is uniform or manifold?

(4) Whether in any of these species the just is the same as counter-passion?

Whether two species of justice are suitably assigned, viz. commutative and distributive?

Objection: 1. It would seem that the two species of justice are unsuitably assigned, viz. distributive and commutative. That which is hurtful to the many cannot be a species of justice, since justice is directed to the common good. Now it is hurtful to the common good of the many, if the goods of the community are distributed among many, both because the goods of the community would be exhausted, and because the morals of men would be corrupted. For Tully says (De Offic. ii, 15): "He who receives becomes worse, and the more ready to expect that he will receive again." Therefore distribution does not belong to any species of justice.
2. Further, the act of justice is to render to each one what is his own, as stated above (Question [58], Article [2]). But when things are distributed, a man does not receive what was his, but becomes possessed of something which belonged to the community. Therefore this does not pertain to justice.
3. Further, justice is not only in the sovereign, but also in the subject, as stated above (Question [58], Article [6]). But it belongs exclusively to the sovereign to distribute. Therefore distribution does not always belong to justice.
4. Further, "Distributive justice regards common goods" (Ethic. v, 4). Now matters regarding the community pertain to legal justice. Therefore distributive justice is a part, not of particular, but of legal justice.
5. Further, unity or multitude do not change the species of a virtue. Now commutative justice consists in rendering something to one person, while distributive justice consists in giving something to many. Therefore they are not different species of justice.

On the contrary The Philosopher assigns two parts to justice and says (Ethic. v, 2) that "one directs distributions, the other, commutations."
I answer that As stated above (Question [58], Articles [7],8), particular justice is directed to the private individual, who is compared to the community as a part to the whole. Now a twofold order may be considered in relation to a part. In the first place there is the order of one part to another, to which corresponds the order of one private individual to another. This order is directed by commutative justice, which is concerned about the mutual dealings between two persons. In the second place there is the order of the whole towards the parts, to which corresponds the order of that which belongs to the community in relation to each single person. This order is directed by distributive justice, which distributes common goods proportionately. Hence there are two species of justice, distributive and commutative.

Reply to Objection: 1. Just as a private individual is praised for moderation in his bounty, and blamed for excess therein, so too ought moderation to be observed in the distribution of common goods, wherein distributive justice directs.
2. Even as part and whole are somewhat the same, so too that which pertains to the whole, pertains somewhat to the part also: so that when the goods of the community are distributed among a number of individuals each one receives that which, in a way, is his own.
3. The act of distributing the goods of the community, belongs to none but those who exercise authority over those goods; and yet distributive justice is also in the subjects to whom those goods are distributed in so far as they are contented by a just distribution. Moreover distribution of common goods is sometimes made not to the state but to the members of a family, and such distribution can be made by authority of a private individual.
4. Movement takes its species from the term "whereunto." Hence it belongs to legal justice to direct to the common good those matters which concern private individuals: whereas on the contrary it belongs to particular justice to direct the common good to particular individuals by way of distribution.
5. Distributive and commutative justice differ not only in respect of unity and multitude, but also in respect of different kinds of due: because common property is due to an individual in one way, and his personal property in another way.

Whether the mean is to be observed in the same way in distributive as in commutative justice?

Objection: 1. It would seem that the mean in distributive justice is to be observed in the same way as in commutative justice. For each of these is a kind of particular justice, as stated above (Article [1]). Now the mean is taken in the same way in all the parts of temperance or fortitude. Therefore the mean should also be observed in the same way in both distributive and commutative justice.
2. Further, the form of a moral virtue consists in observing the mean which is determined in accordance with reason. Since, then, one virtue has one form, it seems that the mean for both should be the same.
3. Further, in order to observe the mean in distributive justice we have to consider the various deserts of persons. Now a person's deserts are considered also in commutative justice, for instance, in punishments; thus a man who strikes a prince is punished more than one who strikes a private individual. Therefore the mean is observed in the same way in both kinds of justice.

On the contrary The Philosopher says (Ethic. v, 3,4) that the mean in distributive justice is observed according to "geometrical proportion," whereas in commutative justice it follows "arithmetical proportion."
I answer that As stated above (Article [1]), in distributive justice something is given to a private individual, in so far as what belongs to the whole is due to the part, and in a quantity that is proportionate to the importance of the position of that part in respect of the whole. Consequently in distributive justice a person receives all the more of the common goods, according as he holds a more prominent position in the community. This prominence in an aristocratic community is gauged according to virtue, in an oligarchy according to wealth, in a democracy according to liberty, and in various ways according to various forms of community. Hence in distributive justice the mean is observed, not according to equality between thing and thing, but according to proportion between things and persons: in such a way that even as one person surpasses another, so that which is given to one person surpasses that which is allotted to another. Hence the Philosopher says (Ethic. v, 3,4) that the mean in the latter case follows "geometrical proportion," wherein equality depends not on quantity but on proportion. For example we say that 6 is to 4 as 3 is to 2, because in either case the proportion equals 1-1/2; since the greater number is the sum of the lesser plus its half: whereas the equality of excess is not one of quantity, because 6 exceeds 4 by 2, while 3 exceeds 2 by 1.On the other hand in commutations something is paid to an individual on account of something of his that has been received, as may be seen chiefly in selling and buying, where the notion of commutation is found primarily. Hence it is necessary to equalize thing with thing, so that the one person should pay back to the other just so much as he has become richer out of that which belonged to the other. The result of this will be equality according to the "arithmetical mean" which is gauged according to equal excess in quantity. Thus 5 is the mean between 6 and 4, since it exceeds the latter and is exceeded by the former, by 1. Accordingly if, at the start, both persons have 5, and one of them receives 1 out of the other's belongings, the one that is the receiver, will have 6, and the other will be left with 4: and so there will be justice if both be brought back to the mean, 1 being taken from him that has 6, and given to him that has 4, for then both will have 5 which is the mean.

Reply to Objection: 1. In the other moral virtues the rational, not the real mean, is to be followed: but justice follows the real mean; wherefore the mean, in justice, depends on the diversity of things.
2. Equality is the general form of justice, wherein distributive and commutative justice agree: but in one we find equality of geometrical proportion, whereas in the other we find equality of arithmetical proportion.
3. In actions and passions a person's station affects the quantity of a thing: for it is a greater injury to strike a prince than a private person. Hence in distributive justice a person's station is considered in itself, whereas in commutative justice it is considered in so far as it causes a diversity of things.

Summa Th. II-II EN Qu.60