Overview
Say's law derives from the "law of markets" set out by the French economist Jean-Baptiste Say in his Traité d'économie politique (1803). Say argued that people produce in order to sell their output and buy other goods, so products are ultimately exchanged for other products. The familiar phrase "supply creates its own demand" is not Say's own wording but Keynes's (1936) summary.
Its place in classical economics
According to Say's law, particular goods may be in surplus for a while, but a general glut, in which all goods in the economy fail to sell at once, cannot last, because prices of surplus goods fall and prices of scarce goods rise until markets adjust. Classical economists such as Mill (1848) accepted the idea and held that recessions were temporary and that markets would return to full employment on their own. Saving, they argued, flows into investment through the interest rate, so demand does not leak away. The reasoning is still often cited in support of policies that raise productivity and business investment.
The general glut controversy and Keynes's critique
In the early nineteenth century Malthus argued against Say and Ricardo that a general overproduction could occur if consumption failed to keep up with production, a debate known as the "general glut controversy".
In The General Theory of Employment, Interest and Money (1936), Keynes attacked Say's law directly. If people want to hold part of their income as money, saving does not automatically become investment, aggregate demand (effective demand) can fall short, and involuntary unemployment can persist. The motives for holding money are covered under liquidity preference, and aggregate-demand management under Keynesian economics.
Limitations and debate
Baumol (1977) pointed out that at least eight distinct propositions have been bundled under the name "Say's law". A weak reading, that productive capacity determines income in the long run, is still widely accepted, but a strong reading, that demand can never be deficient even in the short run, lost credibility after the Great Depression and later financial crises. The distinction often resurfaces in today's debates on supply-side policy, such as the argument over trickle-down economics.
Using it in the history of economics and exam study
Say's law is a staple of comparisons between classical and Keynesian economics in introductory courses, of history-of-economic-thought classes and of civil-service and public-enterprise economics tests. A table contrasting the classical assumptions (flexible prices and wages, saving equals investment) with Keynes's objections (money holding, deficient effective demand, sticky wages) makes an answer easy to organize. For the basics of the demand side, see also the law of demand.
Sources reviewed: 2026-10-16
References
- Say, J.-B. (1803). Traité d'économie politique. Paris: Deterville.
- Mill, J. S. (1848). Principles of Political Economy. London: John W. Parker.
- Keynes, J. M. (1936). The General Theory of Employment, Interest and Money. London: Macmillan.
- Baumol, W. J. (1977). Say's (at least) eight laws, or what Say and James Mill may really have meant. Economica, 44(174), 145–161.