Chapter 5 - Perfectly competitive supply. In our discussion of supply and demand in part 1, we asked you simply to assume the law of demand, which says that demand curves are downward-sloping. In chapter 5 we will see that this law is a simple consequence of the fact that people spend their limited incomes in rational ways. | Lecture Principles of economics 2e Chapter 5 - Robert H. Frank Ben S. Bernanke