ciples 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).]

The difficulty which moderns find in understanding this teaching,
is that it is said to be based on the sterility of money. A moment's
thought, however, will convince us that money is in fact sterile until
labour has been applied to it. In this sense money differs in its
essence from a cow or a tree. A cow will produce calves, or a tree
will produce fruit without the application of any exertion by its
owner; but, whatever profit is derived from money, is derived from the
use to which it is put by the person who owns it. This is all that
the scholastics meant by the sterility of