EXCHANGE
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