at additional wealth should accrue to the owner
of the wealth that produced it.

The fact is, as Boehm-Bawerk has pointed out, that the question of the
productivity of capital was never discussed by the mediaeval schoolmen,
for the simple reason that it was so obvious. The justice of receiving
an income from an infungible thing which was temporarily lent by its
owner, was discussed and supported; but the justice of the owner of
such a thing receiving an income from the thing so long as it remained
in his own possession was never discussed, because it was universally
admitted.[1] It is perfectly correct to say that the problems which
have perplexed modern writers as to the justice of receiving an
unearned income from one's property never occurred to the scholastics;
such problems can only arise when the institution of private property
comes to be questioned; and private property was the keystone of the
whole scholastic economic conception. In other words, the justice of a
reward for capital was admitted because it was unquestioned.

[Footnote 1: _Capital and Interest_, p. 39.]

The question that caused difficulty was whether money could
be considered a form of capital. At the present day, when the
opportunities of industrial investment are wider than they ever were
before, the principal use to which money is put is the financing of
industrial enterprises; but in the Middle Ages this was not the case,
precisely because the opportunities of profitable investment were
so few. This is the reason why the mediaeval writers did not find it
necessary to discuss in detail the rights of the owner of money who
used it for productive purposes. But of the justice of a profit being
reaped when money was actually so employed there was no doubt at all.
As we have seen, the borrower of a sum of money might reap a profit
from its wise employment; there was no question about the justice of
taking such a profit; and the only matter in dispute was whether that
profit should belong to the borrower or the lender of the money. This
dispute was decided in favour of the borrower on the ground that,
according to the true nature of the contract of _mutuum_, the money
was his property. It was, therefore, never doubted that even money
might produce a profit for its owner. The only difference between
infungible goods and money was that, in the case of the former, the
use might be transferred apart from the property, whereas, in the case
of the latt