The scholastic teaching on the subject of the rules of justice in exchange was founded on the famous fifth book of Aristotle's _Ethics_, and is very clearly set forth by Aquinas. In the article of the _Summa_, where the question is discussed, 'Whether the mean is to be observed in the same way in distributive as in commutative justice?' we find a clear exposition: 'In commutations something is delivered 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 equalise 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 are brought back to the mean, I being taken from him that has 6 and given to him that has 4, for then both will have 5, which is the mean.'[1] In the following article the matter of each kind of justice is discussed. We are told that: 'Justice is about certain external operations, namely, distribution and commutation. These consist in the use of certain externals, whether things, persons, or even works: of things as when one man takes from or restores to another that which is his: of persons as when a man does an injury to the very person of another...: and of works as when a man justly enacts a work of another or does a work for him.... Commutative justice directs commutations that can take place between two persons. Of these some are involuntary, some voluntary.... Voluntary commutations are when a man voluntarily transfers his chattel to another person. And if he transfer it simply so that the recipient incurs no debt, as in the case of gifts, it is an act not of justice, but of liberality. A voluntary transfer belongs to justice in so far as it includes the notion of debt.' Aquinas then goes on to distinguish between the different kinds of contract, sale, usufruct, loan, letting and hiring, and deposit, and concludes, 'In all these actions the mean is taken in the same way according to the equality of repayment. Hence all these actions belong to the one species of justice, namely, commutative justice.'[2] [Footnote 1: ii. ii. 61, 2.] [Footnote 2: ii. ii. 61, 3. The reasoning of Aristotle is characteristically reinforced by the quotation of Matt. vii. 12; ii. ii. 77,1.]