, it must be clearly understood that the attempt of some modern
writers to base the mediaeval justification of commerce on an analysis
of all commercial gains as the payment for labour rests on a profound
misunderstanding. As we have already pointed out, Aquinas distinctly
rules out of consideration in his treatment of commerce the case
where the goods have been improved in value by the exertions of the
merchant. When the element of labour entered into the transaction the
matter was clearly beyond doubt, and the lengthy discussion devoted
to the question of commerce by Aquinas and his followers shows that in
justifying commercial gains they were justifying a gain resting not on
the remuneration for the labour, but on an independent title.

Sec. 8. _Cambium_.

There was one department of commerce, namely, _cambium_, or
money-changing, which, while it did not give any difficulty in theory,
involved certain difficulties in practice, owing to the fact that
it was liable to be used to disguise usurious transactions. Although
_cambium_ was, strictly speaking, a special branch of commerce, it was
nevertheless usually treated in the works on usury, the reason being
that many apparent contracts of _cambium_ were in fact veiled loans,
and that it was therefore a matter of importance in discussing usury
to explain the tests by which genuine and usurious exchanges could be
distinguished. Endemann treats this subject very fully and ably;[1]
but for the purpose of the present essay it is not necessary to do
more than to state the main conclusions at which he arrives.

[Footnote 1: _Studien_, vol. i. p. 75.]

Although the practice of exchange grew up slowly and gradually during
the later Middle Ages, and, consequently, the amount of space devoted
to the discussion of the theory of exchange became larger as time went
on, nevertheless there is no serious difference of opinion between
the writers of the thirteenth century, who treat the subject in
a fragmentary way, and those of the fifteenth, who deal with it
exhaustively and systematically. Aquinas does not mention _cambium_
in the _Summa_, but he recognises the necessity for some system of
exchange in the _De Eegimine Principum_.[1] All the later writers who
mention _cambium_ are agreed in regarding it as a species of commerce
to which the ordinary rules regulating all commerce apply. Francis
de Mayronis says that the art of _cambium_ is as natural as any
other kind of commerce,