This definition makes it clear that money, to fulfil its function, must circulate indefinitely, which requires an extremely durable ‘money stuff’. In principle, though not so often in practice, the actual quantity circulating at any one time is fixed, and given the demand for money by those wishing to perform the ritual-whatever its purposes may be-it should be in short supply. It is therefore a pre-eminent instance of *Lévi-Strauss’s (1969
:32) ‘system of the scarce product’ which is described as ‘a model of extreme generality’. Suitable scarce products hardly occur in nature: cowrie shells, used as money in many different parts of the world, are about the only important case. Otherwise money, in the form of coins, tends to be made out of metal.