For this purpose money was invented, and serves as a medium of exchange; for by it we can measure everything, and so can measure the superiority and inferiority of different kinds of work⁠—the number of shoes, for instance, that is equivalent to a house or to a certain quantity of food.

What is needed then is that so many shoes shall bear to a house (or a measure of corn) the same ratio that a builder [or a husbandman] bears to a shoemaker. 113 For unless this adjustment be effected, no dealing or exchange of services can take place; and it cannot be effected unless the things to be exchanged can be in some way made equal.

We want, therefore, some one common measure of value, as we said before.

This measure is, in fact, the need for each other’s services which holds the members of a society together; for if men had no needs, or no common needs, there would either be no exchange, or a different sort of exchange from that which we know.