The common element in diverse products that makes them commensurable is need, or demand (ἡ χρεία), for reciprocal services.[724] But on the basis of the need of the moment, or under the régime of barter, just exchange would be practically impossible, since the concrete needs of A and B, at any given moment, are not likely to correspond. In such a case, exchange would be a gross disregard of the cost of production. This has been avoided by the introduction of money as a substitute for demand,[725] a symbol of general, rather than specific need. Thus just exchange becomes possible, for money, as the representative of general need, is always equally in demand by all, and, as the common denominator of value, it alone renders it possible for proportional amounts of each product to be exchanged.[726] Aristotle’s basal premise in this theory of fair exchange, that unless an equal quantum of value is received by each party, one must lose what the other gains, has been severely criticized by Menger.[727] He objects that the determining consideration in exchange is not the equal value of exchanged goods. On the contrary, men trade only when they expect to better their economic condition. “Um ihres economischen Vortheils willen, nicht um gleiches gegen gleiches hinzugeben; sondern um ihre Bedürfnisse so vollständig als unter den gegebenen Verhältnissen dies zulässig ist zu befriedigen.” Each gives the other only so much of his own goods as is necessary to secure this end, and it is this competition in open market that fixes prices. Barker[728] also criticizes Aristotle on the ground that he takes no account of demand in his theory of just price. He states that if the cost of production were the only element to be considered, the doctrine might be correct, but with the entrance of demand, one may buy at a low price and sell at an advance without injustice.