#### Dubey, P; Sahi, S; Shubik, M

Consider an exchange mechanism which accepts "diversified" offers of various commodities and then redistributes them. Under some natural conditions of "fairness" and "convenience", such a mechanism admits unique prices, which equalize the value of offers and returns for every individual. Next define the complexity of a mechanism via certain integers tau(ij), pi(ij) and k(i) that represent the time required to exchange i for j, the difficulty in determining the exchange ratio, and the dimension of the offers. There are finitely many minimally complex mechanisms, in each of which all trade occurs through markets for commodity pairs. Finally consider minimal mechanisms with smallest worst-case complexities tau = max tau(ij) and pi = max pi(ij) For m > 3 commodities, there are precisely three such mechanisms, one of which has a distinguished commodity - the money - as the sole medium of exchange. As m -> infinity the money mechanism is the only one with bounded (pi, tau).