"The members of an economy—the firms, the consumers, the investors, and the government—make choices. To give a common name to them all, I will refer to them as agents, for indeed their most salient characteristic is that they act. That they make choices implies that they have alternatives, that what was chosen was not inevitable but was in fact only one in a range of opportunities. The opportunities available to a consumer are determined by the income he has and the prices he has to pay for commodities of different use-values."