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, because of the diversity of the currencies in different kingdoms, and approves of the campsor receiving some remuneration for his labour and trouble.[2] Nicholas de Ausmo, in his commentary on the _Summa Pisana_, written in the beginning of the fifteenth century, says that the campsor may receive a gain from his transactions, provided that they are not conducted with the sole object of making a profit, and that the gain he may receive must be limited by the common estimation of the place and time. This is practically saying that _cambium_ may be carried on under the same conditions as any other species of commerce. Biel says that _cambium_ is only legitimate if the campsor has the motive of keeping up a family or benefiting the State, and that the contract may become usurious if the gain is not fair and moderate.[3] The right of the campsor to some remuneration for risk was only gradually admitted, and forms the subject of much discussion amongst the jurists.[4] This hesitation in allowing remuneration for risk was not peculiar to _cambium_, but, as we have seen above, was common to all commerce. Endemann points out how the theologians and jurists unanimously insisted that _cambium_ could not be justified except when the just price was observed, and that, when the doctrine attained its full development, the element of labour was but one of the constituents in the estimation of that price.[5] [Footnote 1: 'Cum enim extraneae monetae communicantur in permutationibus oportet recurrere ad artem campsoriam, cum talia numismata non tantum valeant in regionibus extraneis quantum in propriis (_De Reg. Prin._, ii. 13).] [Footnote 2: In _Quot. Lib. Sent._, iv. 16, 4.] [Footnote 3: _Op. oil_., IV. xv. 11.] [Footnote 4: Endemann, _Studien_, vol. i. pp. 123-36.] [Footnote 5: _Ibid._, p. 213.]