If A stand for a husbandman and C for a certain quantity of his work (or corn), B will stand for a shoemaker, and D for that quantity of shoemaker’s work that is valued as equal to C.

If they could not requite each other in this way, interchange of services would be impossible.

That it is our need which forms, as it were, a common bond to hold society together, is seen from the fact that people do not exchange unless they are in need of one another’s services (each party of the services of the other, or at least one party of the service of the other), as when that which one has, e.g. wine, is needed by other people who offer to export corn in return. This article, then [the corn to be exported], must be made equal [to the wine that is imported]. 117