3. Third Conclusion

120. As to the first transfer, namely exchange of things [n.118], let the conclusion be this (which is the third conclusion of this article): that the owners of things justly exchange them if they preserve, without fraud, the equality of worth in the things exchanged according to right reason - meaning here the conditions for a just giving expounded above [nn.114-115].

121. Explanation of the other conditions that are proper to just exchange [n.120]: As to what is first added, ‘without fraud’, it excludes fraud in substance, and quantity, and quality: in substance, so that brass is not exchanged for gold, nor water for wine; in quantity, namely so that whether quantity is measured by weight or some other measure, namely by a rod or something of the sort as to length or some bodily measure, as namely a sixteenth [of a peck], a peck, or the like, whether in the case of liquid or of dry goods - so that the just weight and, universally, the just measure be kept. Similarly in the case of quality, so that what is received from the other exchanger as pure wine not be exchanged for sour wine. And all these things are proved in Gregory IX, Decretals V tit.36 ch.9, ‘On Injuries and Damage Given’, ‘If by your fault’, “He who gives occasion for a loss, seems to have given a loss.”

122. But he who defrauds in substance him who thinks that, in making the exchange, he is receiving a different substance, or in quantity him who thinks he is receiving a different amount, or in quality him who thinks he is receiving a different quality, gives occasion for loss, because the other would not make the exchange if he did not think he was receiving a different substance, quantity, and quality; therefore he seems, not only according to the book on Refutations [Scotus, Refutations, q.39 nn.5-6, q.40 nn.13-16; Aristotle, Sophistical Refutations 4.166b10-14], but by presumption of law and in truth, to have given a loss.

123. There follows in the rule [n.120; inferred from Ethics 5.8.1133b18] that ‘the equality of worth is to be preserved’, which is proved by Augustine On the Trinity XIII ch.3 n.6, “To want to buy cheaply and to sell dearly is truly a vice.” And I understand this about a cheap and dear thing as concerns use, because often what is in itself a nobler thing in natural being is of less value and less useful for men’s use and, for this reason, less precious, according to Augustine, City of God XI ch.16, “Bread is better in a home than a mouse,” although however a living thing is simply nobler than a non-living thing in being of nature.

124. And for this reason there is added [to the rule, n.120] ‘in accord with right reason’ - namely reason that pays attention to the nature of the thing in relation to the human use for which exchange takes placde.

125. Now this ‘equality in accord with right reason’ [nn.123-124] does not consist in what is indivisible, as a certain doctor says [Henry of Ghent], who was moved by this, that justice keeps only the mean of the thing but the other virtues only the mean of reason; for this is false, as was made clear in Ord. III d.8 nn.58-62 [there citing Henry of Ghent].

126. On the contrary, in this mean, which justice in exchange has regard to, there is considerable latitude, and within this latitude (without reaching the indivisible point of equivalence of thing with thing, because, as far as this is concerned, it would be almost impossible for exchangers to reach it), justice is done to whatever degree it may, as to the extremes, be done.

127. Now what this latitude is, and how far it extends, is sometimes made known by positive law, sometimes by custom. For the law rescinds a contract where a contracting party is deceived above the mean of the just price; however if injustice below that mean appear on the other side, corresponding restitution should be made.

128. But sometimes it is left to the contracting parties themselves so that, after weighing their mutual necessity, they may reckon they are mutually giving and receiving the equivalent on this side and on that; for it is hard for there to be contracts among men where the contracting parties do not intend to return to themselves mutually something of the indivisible injustice, so that, to this extent, some giving away is concomitant to every contract. And if this is the way of people exchanging, which is founded as it were on this remark of the law of nature, ‘Do that unto others which you want to be done to you’ [Matthew 7.12, Luke 6.31; equivalent negative formulation in Tobit 4.16], it is probable enough that when they are mutually content, they wish to make return to themselves mutually, if in any respect they fail of the requisite justice.

129. There is an altogether like conclusion [sc. like the third conclusion, n.120] about justice in buying and selling [n.118], because it is as necessary to consider the coin there on one side as to consider here the thing exchanged.

130. I add that in both these contracts it is licit for the exchanger or seller to weigh his own loss, but not to weigh the gain of the buyer himself, or of the one he is exchanging with - I say this in the case of selling or exchanging at a dearer price.

131. And I understand it as follows: if someone is in great need of his own property and is induced, with great insistence by another, to sell it or exchange it for something else, since he could keep himself without loss and since by this selling or exchanging he suffers a great loss, he can sell it at a higher price than if he were otherwise selling or exchanging it without such a loss.

132. But if the buyer obtain a great advantage from the thing sold or exchanged, it cannot be sold or exchanged [sc. by me] at a higher price because a greater advantage will follow from the thing when sold to him. For neither is my property, because of his greater consequent advantage, more expensive in itself, nor is it better for me, and so it should not bring me a greater price. But it is otherwise when I suffer a loss, because then the thing is more expensive for me, though not in itself.

133. With these contracts, as was said [n.119], agree mutual accommodation, and leasing, and renting out. And justice must in like manner be kept as to the conditions already set down [n.114], considering it in respect of use there as in respect of ownership here.