Sed cum statim res mobiles monstrante necessitate, quae modo explicata est, in ius proprium transissent, inventa est permutatio, qua quod alteri deest ex eo quod alteri superest suppleretur.[151a] Ita commercia victus gratia inventa ex Homero Plinius probat.[152a] Postquam vero res etiam immobiles in dominos distingui coeperunt, sublata undique communio non inter homines locorum spatiis discretos tantum, verum etiam inter vicinos necessarium fecit commercium; quod ut facilius procederet, nummus postea adventus est, dictus ἀπὸ τοῦ νόμου quod institutum sit civile.[153a] Ipsa igitur ratio omnium contractuum universalis, ἡ μεταβλητική a natura est; modi autem aliquot singulares ipsumque pretium, ἡ χρηματιστική ab instituto;[154a] quae vetustiores iuris interpretes non satis distinxerunt. Fatentur - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - and because, as we have said above, everything was not found everywhere, it was necessary to transport things from one place to another; not that there was yet an interchange of commodities, but that people were accustomed to make reciprocal use of things found in one another’s territory according to their own judgment. They say that trade arose among the Chinese in about this way. Things were deposited at places out in the desert and left to the good faith and conscience of those who exchanged things of their own for what they took.[150] But when movables passed into private ownership (a change brought about by necessity, as has been explained above), straightway there arose a method of exchange by which the lack of one person was supplemented by that of which another person had an over supply.[151] Hence commerce was born out of necessity for the commodities of life, as Pliny shows by a citation from Homer.[152] But after immovables also began to be recognized as private property, the consequent annihilation of universal community of use made commerce a necessity not only between men whose habitations were far apart but even between men who were neighbors; and in order that trade might be carried on more easily, somewhat later they invented money, which, as the derivation of the word shows, is a civic institution.[153] Therefore the universal basis of all contracts, namely exchange, is derived from nature; but some particular kinds of exchange, and the money payment itself, are derived from law;[154] although the older commentators on the law have not made this distinction sufficiently clear. Nevertheless all - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -