sale of fungibles, and therefore the just price in such a contract was the return of fungibles of the same value as those lent.  If the particular fungible sold happened to be money, the estimation of the just price was a simple matter—­it was the return of an amount of money of equal value.  As money happened to be the universal measure of value, this simply meant the return of the same amount of money.  Those who maintained that something additional might be claimed for the use of the money lost sight of the fact that the money was incapable of being used apart from its being consumed.[1] To ask for payment for the sale of a thing which not only did not exist, but which was quite incapable of existence, was clearly to ask for something for nothing—­which obviously offended against the first principles of commutative justice.  ‘He that is not bound to lend,’ says Aquinas in another part of the same article, ’may accept repayment for what he has done, but he must not exact more.  Now he is repaid according to equality of justice if he is repaid as much as he lent, wherefore, if he exacts more for the usufruct of a thing which has no other use but the consumption of its substance, he exacts a price of something non-existent, and so his exaction is unjust.’[2] And in the next article the principle that mutuum is a sale appears equally clearly:  ’Money cannot be sold for a greater sum than the amount lent, which has to be paid back.’[3]

[Footnote 1:  Aquinas did not lose sight of the fact that money might, in certain cases, be used apart from being consumed—­for instance, when it was not used as a means of exchange, but as an ornament.  He gives the example of money being sewn up and sealed in a bag to prevent its being spent, and in this condition lent for any purpose.  In this case, of course, the transaction would not be a mutuum, but a locatio et conductio, and therefore a price could be charged for the use of the money (Quaestiones Disputatae de Malo, Q. xiii. art. iv. ad. 15, quoted in Cronin’s Ethics, vol. ii. p. 332).]

[Footnote 2:  II. ii. 78, 1, ad. 5.]

[Footnote 3:  II. ii. 78, 2, ad. 4.  Biel distinguishes three kinds of exchange:  of goods for goods, or barter; of goods for money, or sale; and of money for money; and adds, ’In his contractibus ... generaliter justitia in hoc consistit quod fiant sine fraude, et servetur aequalitas substantiae, qualitatis, quantitatis in commutatis (Op. cit., IV. xv. 1).  Buridan says that usury is contrary to natural law ’ex conditione justitiae quae in aequalitate damni et lucri consistit; quoniam injustum est pro re semel commutata pluries pretium recipere’ (In Lib.  Pol., iv. 6).]