This book is a study in the history of economic thought. It deals with the economics of exchange, that is, with prices, wages and interest rates. Those who pay a certain price or interest rate, or work at a certain wage, may agree to do so because they are in economic need. They are thus in a sense compelled and not free to choose. This problem was first discussed by teachers in the medieval universities. This book follows the discussion from the Middle Ages to the present.