 The commodity theory of money: A classic theory, which goes back all the way to Aristotle (Politics, 1255b–1256b), holds that money is a kind of commodity that fulfills three functions: it serves as (i) a medium of exchange, (ii) a unit of account, and (iii) a store of value. Imagine a society that lacks money, and in which people have to barter goods with each other. Barter only works when there is a double coincidence of wants; that is, when A wants what B has and B wants what A has. But since such coincidences are likely to be uncommon, a barter economy seems both cumbersome and inefficient (Smith 1776, Menger 1892). At some point, people will realize that they can trade more easily if they use some intermediate good—money. This intermediate good should ideally be easy to handle, store and transport (function i). It should be easy to measure and divide to facilitate calculations (function ii). And it should be difficult to destroy so that it lasts over time (function iii).