Aristotle, in both the _Politics_ and the _Ethics_, deals at considerable length with the subject of exchange.[710] He states that it arose out of the natural situation (κατὰ φύσιν) and defines this as “the fact that men had more of some commodities and less of others than they needed.”[711] At first, all exchange was by barter (ἀλλαγή) and there was no trading except for specific need.[712] The development of an international commerce of import and export was made possible by the invention of money. It is this significant fact that furnishes the line of division between the old natural economy and the era of commerce and finance, when exchange and money have become the tools for unlimited individual enrichment.[713] His theory of exchange and just price grows out of his application to exchange of his definition of corrective justice, as a mean between two extremes of injustice.[714] Trade is just when each party to it has the exact equivalent (ἴσον) in value with which he began. Exchange is a mean between profit and loss, which themselves have no proper relation to its true purpose.[715] This does not mean that the traders must receive the same in return (τὸ ἀντιπεπονθὸς κατ᾽ ἰσότητα), but an equivalent, or proportional requital (τὸ ἀντιπεπονθὸς κατ᾽ ἀναλογίαν).[716] It is this fact of proportional requital that makes exchange, and indeed human society, possible.[717] The meaning is illustrated by a proportion in which the producers bear the same relation to each other as their products.[718] By joining means and extremes, the exchangers are brought to a basis of proportional equality (τὸ κατὰ τὴν ἀναλογίαν ἴσον).[719] Thus is determined how many shoes, the shoemaker’s product, must be given for a house, the builder’s product, and the prices of the two commodities are justly settled, with relation to each other.[720] It is very necessary for just exchange, that such proportional equality be effected before the requital or actual transfer takes place. Otherwise one will gain both superiorities (ἀμφοτέρας τὰς ὑπεροχάς), and equality becomes impossible,[721] since the cost of production of things is very diverse.[722] Indeed, the arts themselves could not exist, unless the advantage to the consumer were similar in quantity and quality to the cost to the producer.[723]