Macroeconomists have long debated whether the size of the economy is driven mainly by its capacity to produce goods and services or by the demand for those goods and services. This lesson introduces the supply-focused perspective associated with Say’s law and considers what it can—and cannot—explain.
Say’s law is commonly summarized as “Supply creates its own demand.” The phrase is useful shorthand for a view associated with the early nineteenth-century French economist Jean-Baptiste Say. Historically, Say did not write this exact law, and the slogan simplifies his ideas.
All flashcards
Say’s law
The shorthand idea that production generates income that can support demand for other goods and services.
Keynes’ law
The shorthand idea that demand can determine how much firms produce in the short run, even when they could produce more.
Productive capacity
An economy’s ability to produce output, depending on resources such as labor, capital, technology, and institutions.
Actual output and productive potential
Actual output is what an economy produces; productive potential is what its available capacity allows it to produce.
Why macroeconomics needs both demand and supply
Demand influences short-run production, while productive capacity constrains sustainable output over time.
Quiz questions
What does Keynes’ law emphasize about output in the short run?Answer: Demand can influence how much firms produce, even when the economy has capacity to produce more.
Why did Keynes argue that low output during the Great Depression did not prove that the United States had lost its ability to produce?Answer: Workers, factories, and existing technologies remained available even as firms reduced production.
Why might firms reduce production when demand weakens?Answer: They may expect too few customers to buy additional output.
What is a historical qualification about the slogans used for Say’s law and Keynes’ law?Answer: The slogans are simplifications, not exact sentences written by Say and Keynes.
Why can’t higher demand remove an economy’s long-run supply constraints?Answer: Spending cannot instantly create additional skilled workers, machinery, or technology.
Based on Principles of Economics 3e by Steven A. Greenlaw, David Shapiro, Daniel MacDonald (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.