ay 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 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 prin