Montesquieu

Yet the law of the Visigoths1 inclu­ded one pro­vi­sion favo­ra­ble to com­merce : it requi­red that mer­chants who came from beyond the sea be jud­ged, in the dis­pu­tes that arose bet­ween them, by the laws and by jud­ges of their nation ; this was based on the prac­tice esta­bli­shed among all these inter­mixed peo­ples, that every man should live by his own law, a mat­ter I shall address at length fur­ther on.

Book XI, tit. 3, §2.