Say's law, or the law of markets, in classical economics, states that aggregate production necessarily creates an equal quantity of aggregate demand. The French economist Jean-Baptiste Say (1767–1832) introduced the idea in 1803, in his principal work, A Treatise on Political Economy (Traité d'économie politique):

A product is no sooner created, than it, from that instant, affords a market for other products to the full extent of its own value.

and

As each of us can only purchase the productions of others with his own productions – as the value we can buy is equal to the value we can produce, the more men can produce, the more they will purchase.