The essential thing to notice in this explanation is that the contract of _mutuum_ is shown to be a sale. The distinction between things which are consumed in use (_res fungibiles_), and which are not consumed in use (_res non fungibiles_) was familiar to the civil lawyers; but what they had never perceived was precisely what Aquinas perceived, namely, that the loan of a fungible thing was in fact not a loan at all, but a sale, for the simple reason that the ownership in the thing passed. Once the transaction had been shown to be a sale, the principle of justice to be applied to it became obvious. As we have seen above, in treating of sales, the essential basis of justice in exchange was the observance of _aequalitas_ between buyer and seller--in other words, the fixing of a just price. The contract of _mutuum_, however, was nothing else than a 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.]