Summa Th. II-II EN Qu.74 a.2

Whether backbiting is a graver sin than tale-bearing?

Objection: 1. It would seem that backbiting is a graver sin than tale-bearing. For sins of word consist in speaking evil. Now a backbiter speaks of his neighbor things that are evil simply, for such things lead to the loss or depreciation of his good name: whereas a tale-bearer is only intent on saying what is apparently evil, because to wit they are unpleasant to the hearer. Therefore backbiting is a graver sin than tale-bearing.
2. Further, he that deprives. a man of his good name, deprives him not merely of one friend, but of many, because everyone is minded to scorn the friendship of a person with a bad name. Hence it is reproached against a certain individual [*King Josaphat] (2Ch 19,2): "Thou art joined in friendship with them that hate the Lord." But tale-bearing deprives one of only one friend. Therefore backbiting is a graver sin than tale-bearing.
3. Further, it is written (Jc 4,11): "He that backbiteth [Douay:,'detracteth'] his brother . . . detracteth the law," and consequently God the giver of the law. Wherefore the sin of backbiting seems to be a sin against God, which is most grievous, as stated above (Question [20], Article [3]; FS, Question [73], Article [3]). On the other hand the sin of tale-bearing is against one's neighbor. Therefore the sin of backbiting is graver than the sin of tale-bearing.

On the contrary It is written (Si 5,17): "An evil mark of disgrace is upon the double-tongued; but to the tale-bearer [Douay: 'whisperer'] hatred, and enmity, and reproach."
I answer that As stated above (Question [73], Article [3]; FS, Question [73], Article [8]), sins against one's neighbor are the more grievous, according as they inflict a greater injury on him: and an injury is so much the greater, according to the greatness of the good which it takes away. Now of all one's external goods a friend takes the first place, since "no man can live without friends," as the Philosopher declares (Ethic. viii, 1). Hence it is written (Si 6,15): "Nothing can be compared to a faithful friend." Again, a man's good name whereof backbiting deprives him, is most necessary to him that he may be fitted for friendship. Therefore tale-bearing is a greater sin than backbiting or even reviling, because a friend is better than honor, and to be loved is better than to be honored, according to the Philosopher (Ethic. viii).

Reply to Objection: 1. The species and gravity of a sin depend on the end rather than on the material object, wherefore, by reason of its end, tale-bearing is worse than backbiting, although sometimes the backbiter says worse things.
2. A good name is a disposition for friendship, and a bad name is a disposition for enmity. But a disposition falls short of the thing for which it disposes. Hence to do anything that leads to a disposition for enmity is a less grievous sin than to do what conduces directly to enmity.
3. He that backbites his brother, seems to detract the law, in so far as he despises the precept of love for one's neighbor: while he that strives to sever friendship seems to act more directly against this precept. Hence the latter sin is more specially against God, because "God is charity" (1Jn 4,16), and for this reason it is written (Pr 6,16): "Six things there are, which the Lord hateth, and the seventh His soul detesteth," and the seventh is "he (Pr 6,19) that soweth discord among brethren."


We must now speak of derision, under which head there are two points of inquiry:

(1) Whether derision is a special sin distinct from the other sins whereby one's neighbor is injured by words?

(2) Whether derision is a mortal sin?

Whether derision is a special sin distinct from those already mentioned?

Objection: 1. It would seem that derision is not a special sin distinct from those mentioned above. For laughing to scorn is apparently the same as derision. But laughing to scorn pertains to reviling. Therefore derision would seem not to differ from reviling.
2. Further, no man is derided except for something reprehensible which puts him to shame. Now such are sins; and if they be imputed to a person publicly, it is a case of reviling, if privately, it amounts to backbiting or tale-bearing. Therefore derision is not distinct from the foregoing vices.
3. Further, sins of this kind are distinguished by the injury they inflict on one's neighbor. Now the injury inflicted on a man by derision affects either his honor, or his good name, or is detrimental to his friendship. Therefore derision is not a sin distinct from the foregoing.

On the contrary Derision is done in jest, wherefore it is described as "making fun." Now all the foregoing are done seriously and not in jest. Therefore derision differs from all of them.
I answer that As stated above (Question [72], Article [2]), sins of word should be weighed chiefly by the intention of the speaker, wherefore these sins are differentiated according to the various intentions of those who speak against another. Now just as the railer intends to injure the honor of the person he rails, the backbiter to depreciate a good name, and the tale-bearer to destroy friendship, so too the derider intends to shame the person he derides. And since this end is distinct from the others, it follows that the sin of derision is distinct from the foregoing sins.

Reply to Objection: 1. Laughing to scorn and derision agree as to the end but differ in mode, because derision is done with the "mouth," i.e. by words and laughter, while laughing to scorn is done by wrinkling the nose, as a gloss says on Ps 2,4, "He that dwelleth in heaven shall laugh at them": and such a distinction does not differentiate the species. Yet they both differ from reviling, as being shamed differs from being dishonored: for to be ashamed is "to fear dishonor," as Damascene states (De Fide Orth. ii, 15).
2. For doing a virtuous deed a man deserves both respect and a good name in the eyes of others, and in his own eyes the glory of a good conscience, according to 2Co 1,12, "Our glory is this, the testimony of our conscience." Hence, on the other hand, for doing a reprehensible, i.e. a vicious action, a man forfeits his honor and good name in the eyes of others---and for this purpose the reviler and the backbiter speak of another person---while in his own eyes, he loses the glory of his conscience through being confused and ashamed at reprehensible deeds being imputed to him---and for this purpose the derider speaks ill of him. It is accordingly evident that derision agrees with the foregoing vices as to the matter but differs as to the end.
3. A secure and calm conscience is a great good, according to Pr 15,15, "A secure mind is like a continual feast." Wherefore he that disturbs another's conscience by confounding him inflicts a special injury on him: hence derision is a special kind of sin.

Whether derision can be a mortal sin?

Objection: 1. It would seem that derision cannot be a mortal sin. Every mortal sin is contrary to charity. But derision does not seem contrary to charity, for sometimes it takes place in jest among friends, wherefore it is known as "making fun." Therefore derision cannot be a mortal sin.
2. Further, the greatest derision would appear to be that which is done as an injury to God. But derision is not always a mortal sin when it tends to the injury of God: else it would be a mortal sin to relapse into a venial sin of which one has repented. For Isidore says (De Sum. Bon. ii, 16) that "he who continues to do what he has repented of, is a derider and not a penitent." It would likewise follow that all hypocrisy is a mortal sin, because, according to Gregory (Moral. xxxi, 15) "the ostrich signifies the hypocrite, who derides the horse, i.e. the just man, and his rider, i.e. God." Therefore derision is not a mortal sin.
3. Further, reviling and backbiting seem to be graver sins than derision, because it is more to do a thing seriously than in jest. But not all backbiting or reviling is a mortal sin. Much less therefore is derision a mortal sin.

On the contrary It is written (Pr 3,34): "He derideth [Vulg.: 'shall scorn'] the scorners." But God's derision is eternal punishment for mortal sin, as appears from the words of Ps 2,4, "He that dwelleth in heaven shall laugh at them." Therefore derision is a mortal sin.
I answer that The object of derision is always some evil or defect. Now when an evil is great, it is taken, not in jest, but seriously: consequently if it is taken in jest or turned to ridicule (whence the terms 'derision' and 'jesting'), this is because it is considered to be slight. Now an evil may be considered to be slight in two ways: first, in itself, secondly, in relation to the person. When anyone makes game or fun of another's evil or defect, because it is a slight evil in itself, this is a venial sin by reason of its genus. on the other hand this defect may be considered as a slight evil in relation to the person, just as we are wont to think little of the defects of children and imbeciles: and then to make game or fun of a person, is to scorn him altogether, and to think him so despicable that his misfortune troubles us not one whit, but is held as an object of derision. In this way derision is a mortal sin, and more grievous than reviling, which is also done openly: because the reviler would seem to take another's evil seriously; whereas the derider does so in fun, and so would seem the more to despise and dishonor the other man. Wherefore, in this sense, derision is a grievous sin, and all the more grievous according as a greater respect is due to the person derided.Consequently it is an exceedingly grievous sin to deride God and the things of God, according to Is 37,23, "Whom hast thou reproached, and whom hast thou blasphemed, and against whom hast thou exalted thy voice?" and he replies: "Against the Holy One of Israel." In the second place comes derision of one's parents, wherefore it is written (Pr 30,17): "The eye that mocketh at his father, and that despiseth the labor of his mother in bearing him, let the ravens of the brooks pick it out, and the young eagles eat it." Further, the derision of good persons is grievous, because honor is the reward of virtue, and against this it is written (Jb 12,4): "The simplicity of the just man is laughed to scorn." Such like derision does very much harm: because it turns men away from good deeds, according to Gregory (Moral. xx, 14), "Who when they perceive any good points appearing in the acts of others, directly pluck them up with the hand of a mischievous reviling."

Reply to Objection: 1. Jesting implies nothing contrary to charity in relation to the person with whom one jests, but it may imply something against charity in relation to the person who is the object of the jest, on account of contempt, as stated above.
2. Neither he that relapses into a sin of which he has repented, nor a hypocrite, derides God explicitly, but implicitly, in so far as either's behavior is like a derider's. Nor is it true that to commit a venial sin is to relapse or dissimulate altogether, but only dispositively and imperfectly.
3. Derision considered in itself is less grievous than backbiting or reviling, because it does not imply contempt, but jest. Sometimes however it includes greater contempt than reviling does, as stated above, and then it is a grave sin.


We must now consider cursing. Under this head there are four points of inquiry:

(1) Whether one may lawfully curse another?

(2) Whether one may lawfully curse an irrational creature?

(3) Whether cursing is a mortal sin?

(4) Of its comparison with other sins.

Whether it is lawful to curse anyone?

Objection: 1. It would seem unlawful to curse anyone. For it is unlawful to disregard the command of the Apostle in whom Christ spoke, according to 2Co 13,3. Now he commanded (Rm 12,14), "Bless and curse not." Therefore it is not lawful to curse anyone.
2. Further, all are bound to bless God, according to Da 3,82, "O ye sons of men, bless the Lord." Now the same mouth cannot both bless God and curse man, as proved in the third chapter of James. Therefore no man may lawfully curse another man.
3. Further, he that curses another would seem to wish him some evil either of fault or of punishment, since a curse appears to be a kind of imprecation. But it is not lawful to wish ill to anyone, indeed we are bound to pray that all may be delivered from evil. Therefore it is unlawful for any man to curse.
4. Further, the devil exceeds all in malice on account of his obstinacy. But it is not lawful to curse the devil, as neither is it lawful to curse oneself; for it is written (Si 21,30): "While the ungodly curseth the devil, he curseth his own soul." Much less therefore is it lawful to curse a man.
5. Further, a gloss on Nb 23,8, "How shall I curse whom God hath not cursed?" says: "There cannot be a just cause for cursing a sinner if one be ignorant of his sentiments." Now one man cannot know another man's sentiments, nor whether he is cursed by God. Therefore no man may lawfully curse another.

On the contrary It is written (Dt 27,26): "Cursed be he that abideth not in the words of this law." Moreover Eliseus cursed the little boys who mocked him (2R 2,24).
I answer that To curse [maledicere] is the same as to speak ill [malum dicere]. Now "speaking" has a threefold relation to the thing spoken. First, by way of assertion, as when a thing is expressed in the indicative mood: in this way "maledicere" signifies simply to tell someone of another's evil, and this pertains to backbiting, wherefore tellers of evil [maledici] are sometimes called backbiters. Secondly, speaking is related to the thing spoken, by way of cause, and this belongs to God first and foremost, since He made all things by His word, according to Ps 32,9, "He spoke and they were made"; while secondarily it belongs to man, who, by his word, commands others and thus moves them to do something: it is for this purpose that we employ verbs in the imperative mood. Thirdly, "speaking" is related to the thing spoken by expressing the sentiments of one who desires that which is expressed in words; and for this purpose we employ the verb in the optative mood.Accordingly we may omit the first kind of evil speaking which is by way of simple assertion of evil, and consider the other two kinds. And here we must observe that to do something and to will it are consequent on one another in the matter of goodness and wickedness, as shown above (FS, Question [20], Article [3]). Hence in these two ways of evil speaking, by way of command and by way of desire, there is the same aspect of lawfulness and unlawfulness, for if a man commands or desires another's evil, as evil, being intent on the evil itself, then evil speaking will be unlawful in both ways, and this is what is meant by cursing. On the other hand if a man commands or desires another's evil under the aspect of good, it is lawful; and it may be called cursing, not strictly speaking, but accidentally, because the chief intention of the speaker is directed not to evil but to good.Now evil may be spoken, by commanding or desiring it, under the aspect of a twofold good. Sometimes under the aspect of just, and thus a judge lawfully curses a man whom he condemns to a just penalty: thus too the Church curses by pronouncing anathema. In the same way the prophets in the Scriptures sometimes call down evils on sinners, as though conforming their will to Divine justice, although such like imprecation may be taken by way of foretelling. Sometimes evil is spoken under the aspect of useful, as when one wishes a sinner to suffer sickness or hindrance of some kind, either that he may himself reform, or at least that he may cease from harming others.

Reply to Objection: 1. The Apostle forbids cursing strictly so called with an evil intent:
2. and the same answer applies to the Second Objection.
3. To wish another man evil under the aspect of good, is not opposed to the sentiment whereby one wishes him good simply, in fact rather is it in conformity therewith.
4. In the devil both nature and guilt must be considered. His nature indeed is good and is from God nor is it lawful to curse it. On the other hand his guilt is deserving of being cursed, according to Jb 3,8, "Let them curse it who curse the day." Yet when a sinner curses the devil on account of his guilt, for the same reason he judges himself worthy of being cursed; and in this sense he is said to curse his own soul.
5. Although the sinner's sentiments cannot be perceived in themselves, they can be perceived through some manifest sin, which has to be punished. Likewise although it is not possible to know whom God curses in respect of final reprobation, it is possible to know who is accursed of God in respect of being guilty of present sin.

Whether it is lawful to curse an irrational creature?

Objection: 1. It would seem that it is unlawful to curse an irrational creature. Cursing would seem to be lawful chiefly in its relation to punishment. Now irrational creatures are not competent subjects either of guilt or of punishment. Therefore it is unlawful to curse them.
2. Further, in an irrational creature there is nothing but the nature which God made. But it is unlawful to curse this even in the devil, as stated above (Article [1]). Therefore it is nowise lawful to curse an irrational creature.
3. Further, irrational creatures are either stable, as bodies, or transient, as the seasons. Now, according to Gregory (Moral. iv, 2), "it is useless to curse what does not exist, and wicked to curse what exists." Therefore it is nowise lawful to curse an irrational creature.

On the contrary our Lord cursed the fig tree, as related in Mt 21,19; and Job cursed his day, according to Jb 3,1.
I answer that Benediction and malediction, properly speaking, regard things to which good or evil may happen, viz. rational creatures: while good and evil are said to happen to irrational creatures in relation to the rational creature for whose sake they are. Now they are related to the rational creature in several ways. First by way of ministration, in so far as irrational creatures minister to the needs of man. In this sense the Lord said to man (Gn 3,17): "Cursed is the earth in thy work," so that its barrenness would be a punishment to man. Thus also David cursed the mountains of Gelboe, according to Gregory's expounding (Moral. iv, 3). Again the irrational creature is related to the rational creature by way of signification: and thus our Lord cursed the fig tree in signification of Judea. Thirdly, the irrational creature is related to rational creatures as something containing them, namely by way of time or place: and thus Job cursed the day of his birth, on account of the original sin which he contracted in birth, and on account of the consequent penalties. In this sense also we may understand David to have cursed the mountains of Gelboe, as we read in 2S 1,21, namely on account of the people slaughtered there.But to curse irrational beings, considered as creatures of God, is a sin of blasphemy; while to curse them considered in themselves is idle and vain and consequently unlawful.

Reply to Objection: 1. From this the Replies to the objections may easily be gathered.

Whether cursing is a mortal sin?

Objection: 1. It would seem that cursing is not a mortal sin. For Augustine in a homily On the Fire of Purgatory [*Serm. civ in the appendix of St. Augustine's works] reckons cursing among slight sins. But such sins are venial. Therefore cursing is not a mortal but a venial Sin.
2. Further, that which proceeds from a slight movement of the mind does not seem to be generically a mortal sin. But cursing sometimes arises from a slight movement. Therefore cursing is not a mortal sin.
3. Further, evil deeds are worse than evil words. But evil deeds are not always mortal sins. Much less therefore is cursing a mortal sin.

On the contrary Nothing save mortal sin excludes one from the kingdom of God. But cursing excludes from the kingdom of God, according to 1Co 6,10, "Nor cursers [Douay: 'railers'], nor extortioners shall possess the kingdom of God." Therefore cursing is a mortal sin.
I answer that The evil words of which we are speaking now are those whereby evil is uttered against someone by way of command or desire. Now to wish evil to another man, or to conduce to that evil by commanding it, is, of its very nature, contrary to charity whereby we love our neighbor by desiring his good. Consequently it is a mortal sin, according to its genus, and so much the graver, as the person whom we curse has a greater claim on our love and respect. Hence it is written (Lv 20,9): "He that curseth his father, or mother, dying let him die."It may happen however that the word uttered in cursing is a venial sin either through the slightness of the evil invoked on another in cursing him, or on account of the sentiments of the person who utters the curse; because he may say such words through some slight movement, or in jest, or without deliberation, and sins of word should be weighed chiefly with regard to the speaker's intention, as stated above (Question [72], Article [2]).

Reply to Objection: 1. From this the Replies to the Objections may be easily gathered.

Whether cursing is a graver sin than backbiting?

Objection: 1. It would seem that cursing is a graver sin than backbiting. Cursing would seem to be a kind of blasphemy, as implied in the canonical epistle of Jude (verse 9) where it is said that "when Michael the archangel, disputing with the devil, contended about the body of Moses, he durst not bring against him the judgment of blasphemy [Douay: 'railing speech']," where blasphemy stands for cursing, according to a gloss. Now blasphemy is a graver sin than backbiting. Therefore cursing is a graver sin than backbiting.
2. Further, murder is more grievous than backbiting, as stated above (Question [73], Article [3]). But cursing is on a par with the sin of murder; for Chrysostom says (Hom. xix, super Matth.): "When thou sayest: 'Curse him down with his house, away with everything,' you are no better than a murderer." Therefore cursing is graver than backbiting.
3. Further, to cause a thing is more than to signify it. But the curser causes evil by commanding it, whereas the backbiter merely signifies an evil already existing. Therefore the curser sins more grievously than the backbiter.

On the contrary It is impossible to do well in backbiting, whereas cursing may be either a good or an evil deed, as appears from what has been said (Article [1]). Therefore backbiting is graver than cursing.
I answer that As stated in the FP, Question [48], Article [5], evil is twofold, evil of fault, and evil of punishment; and of the two, evil of fault is the worse (FP, Question [48], Article [6]). Hence to speak evil of fault is worse than to speak evil of punishment, provided the mode of speaking be the same. Accordingly it belongs to the reviler, the tale-bearer, the backbiter and the derider to speak evil of fault, whereas it belongs to the evil-speaker, as we understand it here, to speak evil of punishment, and not evil of fault except under the aspect of punishment. But the mode of speaking is not the same, for in the case of the four vices mentioned above, evil of fault is spoken by way of assertion, whereas in the case of cursing evil of punishment is spoken, either by causing it in the form of a command, or by wishing it. Now the utterance itself of a person's fault is a sin, in as much as it inflicts an injury on one's neighbor, and it is more grievous to inflict an injury, than to wish to inflict it, other things being equal.Hence backbiting considered in its generic aspect is a graver sin than the cursing which expresses a mere desire; while the cursing which is expressed by way of command, since it has the aspect of a cause, will be more or less grievous than backbiting, according as it inflicts an injury more or less grave than the blackening of a man's good name. Moreover this must be taken as applying to these vices considered in their essential aspects: for other accidental points might be taken into consideration, which would aggravate or extenuate the aforesaid vices.

Reply to Objection: 1. To curse a creature, as such, reflects on God, and thus accidentally it has the character of blasphemy; not so if one curse a creature on account of its fault: and the same applies to backbiting.
2. As stated above (Article [3]), cursing, in one way, includes the desire for evil, where if the curser desire the evil of another's violent death, he does not differ, in desire, from a murderer, but he differs from him in so far as the external act adds something to the act of the will.
3. This argument considers cursing by way of command.



We must now consider those sins which relate to voluntary commutations. First, we shall consider cheating, which is committed in buying and selling: secondly, we shall consider usury, which occurs in loans. In connection with the other voluntary commutations no special kind of sin is to be found distinct from rapine and theft.

Under the first head there are four points of inquiry:

(1) Of unjust sales as regards the price; namely, whether it is lawful to sell a thing for more than its worth?

(2) Of unjust sales on the part of the thing sold;

(3) Whether the seller is bound to reveal a fault in the thing sold?

(4) Whether it is lawful in trading to sell a thing at a higher price than was paid for it?

Whether it is lawful to sell a thing for more than its worth?

Objection: 1. It would seem that it is lawful to sell a thing for more than its worth. In the commutations of human life, civil laws determine that which is just. Now according to these laws it is just for buyer and seller to deceive one another (Cod. IV, xliv, De Rescind. Vend. 8,15): and this occurs by the seller selling a thing for more than its worth, and the buyer buying a thing for less than its worth. Therefore it is lawful to sell a thing for more than its worth
2. Further, that which is common to all would seem to be natural and not sinful. Now Augustine relates that the saying of a certain jester was accepted by all, "You wish to buy for a song and to sell at a premium," which agrees with the saying of Pr 20,14, "It is naught, it is naught, saith every buyer: and when he is gone away, then he will boast." Therefore it is lawful to sell a thing for more than its worth.
3. Further, it does not seem unlawful if that which honesty demands be done by mutual agreement. Now, according to the Philosopher (Ethic. viii, 13), in the friendship which is based on utility, the amount of the recompense for a favor received should depend on the utility accruing to the receiver: and this utility sometimes is worth more than the thing given, for instance if the receiver be in great need of that thing, whether for the purpose of avoiding a danger, or of deriving some particular benefit. Therefore, in contracts of buying and selling, it is lawful to give a thing in return for more than its worth.

On the contrary It is written (Mt 7,12): "All things . . . whatsoever you would that men should do to you, do you also to them." But no man wishes to buy a thing for more than its worth. Therefore no man should sell a thing to another man for more than its worth.
I answer that It is altogether sinful to have recourse to deceit in order to sell a thing for more than its just price, because this is to deceive one's neighbor so as to injure him. Hence Tully says (De Offic. iii, 15): "Contracts should be entirely free from double-dealing: the seller must not impose upon the bidder, nor the buyer upon one that bids against him."But, apart from fraud, we may speak of buying and selling in two ways. First, as considered in themselves, and from this point of view, buying and selling seem to be established for the common advantage of both parties, one of whom requires that which belongs to the other, and vice versa, as the Philosopher states (Polit. i, 3). Now whatever is established for the common advantage, should not be more of a burden to one party than to another, and consequently all contracts between them should observe equality of thing and thing. Again, the quality of a thing that comes into human use is measured by the price given for it, for which purpose money was invented, as stated in Ethic. v, 5. Therefore if either the price exceed the quantity of the thing's worth, or, conversely, the thing exceed the price, there is no longer the equality of justice: and consequently, to sell a thing for more than its worth, or to buy it for less than its worth, is in itself unjust and unlawful.Secondly we may speak of buying and selling, considered as accidentally tending to the advantage of one party, and to the disadvantage of the other: for instance, when a man has great need of a certain thing, while an other man will suffer if he be without it. In such a case the just price will depend not only on the thing sold, but on the loss which the sale brings on the seller. And thus it will be lawful to sell a thing for more than it is worth in itself, though the price paid be not more than it is worth to the owner. Yet if the one man derive a great advantage by becoming possessed of the other man's property, and the seller be not at a loss through being without that thing, the latter ought not to raise the price, because the advantage accruing to the buyer, is not due to the seller, but to a circumstance affecting the buyer. Now no man should sell what is not his, though he may charge for the loss he suffers.On the other hand if a man find that he derives great advantage from something he has bought, he may, of his own accord, pay the seller something over and above: and this pertains to his honesty.

Objection: 1. As stated above (FS, Question [96], Article [2]) human law is given to the people among whom there are many lacking virtue, and it is not given to the virtuous alone. Hence human law was unable to forbid all that is contrary to virtue; and it suffices for it to prohibit whatever is destructive of human intercourse, while it treats other matters as though they were lawful, not by approving of them, but by not punishing them. Accordingly, if without employing deceit the seller disposes of his goods for more than their worth, or the buyer obtain them for less than their worth, the law looks upon this as licit, and provides no punishment for so doing, unless the excess be too great, because then even human law demands restitution to be made, for instance if a man be deceived in regard to more than half the amount of the just price of a thing [*Cod. IV, xliv, De Rescind. Vend. 2,8].On the other hand the Divine law leaves nothing unpunished that is contrary to virtue. Hence, according to the Divine law, it is reckoned unlawful if the equality of justice be not observed in buying and selling: and he who has received more than he ought must make compensation to him that has suffered loss, if the loss be considerable. I add this condition, because the just price of things is not fixed with mathematical precision, but depends on a kind of estimate, so that a slight addition or subtraction would not seem to destroy the equality of justice.
2. As Augustine says "this jester, either by looking into himself or by his experience of others, thought that all men are inclined to wish to buy for a song and sell at a premium. But since in reality this is wicked, it is in every man's power to acquire that justice whereby he may resist and overcome this inclination." And then he gives the example of a man who gave the just price for a book to a man who through ignorance asked a low price for it. Hence it is evident that this common desire is not from nature but from vice, wherefore it is common to many who walk along the broad road of sin.
3. In commutative justice we consider chiefly real equality. On the other hand, in friendship based on utility we consider equality of usefulness, so that the recompense should depend on the usefulness accruing, whereas in buying it should be equal to the thing bought.

Summa Th. II-II EN Qu.74 a.2