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
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