’In order to make this evident, we must observe that there are certain things the use of which consists in their consumption; thus we consume wine when we use it for drink, and we consume wheat when we use it for food.  Wherefore in such-like things the use of the thing must not be reckoned apart from the thing itself, and whoever is granted the use of the thing is granted the thing itself; and for this reason to lend things of this kind is to transfer the ownership.  Accordingly, if a man wanted to sell wine separately from the use of the wine, he would be selling the same thing twice, or he would be selling what does not exist, wherefore he would evidently commit a sin of injustice.  In like manner he commits an injustice who lends wine or wheat, and asks for double payment, viz. one, the return of the thing in equal measure, the other, the price of the use, which is called usury.

’On the other hand, there are other things the use of which does not consist in their consumption; thus to use a house is to dwell in it, not to destroy it.  Wherefore in such things both may be granted; for instance, one man may hand over to another the ownership of his house, while reserving to himself the use of it for a time, or, vice versa, he may grant the use of a house while retaining the ownership.  For this reason a man may lawfully make a charge for the use of his house, and, besides this, revendicate the house from the person to whom he has granted its use, as happens in renting and letting a house.

’But money, according to the philosopher,[1] was invented chiefly for the purpose of exchange; and consequently the proper and principal use of money is its consumption or alienation, whereby it is sunk in exchange.  Hence it is by its very nature unlawful to take payment for the use of money lent, which payment is known as usury; and, just as a man is bound to restore other ill-gotten goods, so he is bound to restore the money which he has taken in usury.’[2]

[Footnote 1:  Eth. v. Pol. 1.]

[Footnote 2:  II. ii. 78, 1.]

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