In economics, the principle that total supply creates enough income to purchase total output is called Say's law.
The idea is associated with French economist Jean-Baptiste Say and his 1803 work A Treatise on Political Economy. In its classical formulation, production generates income for the factors used in production, and that income provides purchasing power for goods and services.
Say's law became central to nineteenth-century debates about whether economies could suffer from general overproduction. John Maynard Keynes later criticized the idea, arguing that total demand can fall short of total output when income is saved rather than spent at a sufficient rate. Keynesian economics therefore emphasizes the possibility of economy-wide demand deficiencies.
The phrase is often shortened to “supply creates its own demand,” but that wording can oversimplify Say's argument. It does not mean every individual product automatically sells, nor does it deny sector-specific gluts. The controversy concerns aggregate output and demand across the economy.