Even in the most primitive societies the great majority of people satisfy a large part of their material needs by exchanging goods and services. Very few people indeed can make for themselves everything they need—all their food, their clothes, their housing, their tools. Ever since men started living in communities, they have been satisfying their needs by means of specialization and exchange; increasingly each individual has concentrated on what he can do best, and has produced more of the special goods or services in which he has concentrated, than he can consume himself. The surplus he has exchanged with other members of the community, acquiring, in exchange the things he needs that others have produced. Exchange of goods becomes possible only when