All the writers who treated of exchange divided it into three kinds; ordinary exchange of the moneys of different currencies (_cambium minutum_), exchange of moneys of different currencies between different places, the justification for which rested on remuneration for an imaginary transport (_cambium per litteras_), and usurious exchange of moneys of the same currency (_cambium siccum_). The former two species of cambium were justifiable, whereas the last was condemned.[1] [Footnote 1: Laurentius de Rodulfis, _De Usuris_, pt. iii. Nos. 1 to 5.] The most complete treatise on the subject of money exchange is that of Thomas da Vio, written in 1499. The author of this treatise divides money-changing into three kinds, just, unjust, and doubtful. There were three kinds of just change; _cambium minutum_, in which the campsor was entitled to a reasonable remuneration for his labour; _cambium per litteras_, in which the campsor was held entitled to a wage (_merces_) for an imaginary transportation; and thirdly, when the campsor carried money from one place to another, where it was of higher value. The unjust change was when the contract was a usurious transaction veiled in the guise of a genuine exchange. Under the doubtful changes, the author discusses various special points which need not detain us here. Thomas da Vio then goes on to discuss whether the justifiable exchange can be said to be a species of loan, and concludes that it can not, because all that the campsor receives is an indemnity against loss and a remuneration for his labour, trouble, outlay, and risk, which is always justifiable. He then goes on to state the very important principle, that in _cambium_ money is not to be considered a measure of value, but a vendible commodity,[1] a distinction which Endemann thinks was productive of very important results in the later teaching on the subject.[2] The last question treated in the treatise is the measure of the campsor's profit, and here the contract of exchange is shown to be on all fours with every other contract, because the essential principle laid down for determining its justice is the observance of the equivalence between both parties.[1] [Footnote 1: 'Numisma quamvis sit mensura et instrumentum in permutationibus; tamen per se aliquid esse potest.' It is this principle that justifies the treatment of _cambium_ in this section rather than the next.] [Footnote 2: _Studien_, vol. ii. p. 212.] SECTION 2.--THE SALE OF THE USE OF MONEY § 1. _Usury in Greece and Rome_.