Within The Wealth of a Nation, Smith attributes the wealth of the nation to the self-interest of individuals. Adam Smith states that self-interest is what controls people's behavior and that the economy is "guided by an invisible hand to promote an end which is no part of its intention" (The Wealth of Nations, Book 1, Chapter 7). For example, a drug dealer does not sell drugs based on good intentions. Instead, a drug dealer sells drugs because it makes him a profit. If the drug dealer starts selling low-quality drugs at an unreasonable price, people will not buy drugs from the drug dealer. Therefore, to retain customers, it is in the drug dealer's best interest to sell quality drugs at a reasonable price. Therefore, self-interest is beneficial to all members of society. In the drug dealer scenario, the dealer generates a profit and the customer increases their utility from the drug. Smith measures wealth as a flow of goods and services, based on the productivity of labor. Smith postulates that workers are more productive when there is a division of workers and they engage in specialization. An increase in labor productivity increases a firm's output and leads to market expansion. To support market expansion, Smith suggests investing in machinery to make workers more productive. Eventually, the firm will raise capital