The most famous tenet of Say's law is 'demand creates its own supply'.

In economics, demand is the quantity of a good or service that consumers are willing and able to buy at a given price in a specific period. Supply, on the other hand, refers to the quantity of a good or service that producers are willing and able to sell at a given price in a specific period. Demand and supply form the basis of market dynamics, determining the price at which goods and services are traded.

Say's law, proposed by French economist Jean-Baptiste Say, argues that the production of a good or service generates the necessary demand for that good or service. In other words, by creating an output, producers are also creating the purchasing power for consumers to buy that output. It implies that in the aggregate, supply will always equal demand and the economy will always be at full employment.

According to Say's law, the production of goods and services generates income for the providers of factors of production, such as labor, capital, land, and entrepreneurship. This income, when spent, creates demand for goods and services. So, when a company produces a good or service, it is also generating income and purchasing power for its workers, suppliers, and other stakeholders. This purchasing power then generates demand for other goods and services in the economy.

If we accept Say's law, in the long run, there would be no shortage of demand in the economy since producing goods and services will generate enough income to purchase these goods and services. This implies that recession or unemployment would only be the result of temporary mismatches between the skills and resources available and the types of goods and services demanded by consumers, or other short-term factors.

Say's law has been criticized for its oversimplification of the complex relationship between aggregate demand and aggregate supply in the economy. Critics argue that increased production does not always lead to increased demand for the same goods, especially in cases of overproduction. Modern theories, such as Keynesian economics, focus on the importance of aggregate demand in determining economic activity and suggest that governments can use fiscal and monetary policies to stimulate demand and help alleviate unemployment and recession. In conclusion, Say's law offers an interesting perspective on the relationship between demand and supply. While it highlights an essential connection between production and income, it does not provide a complete explanation for the economic fluctuations and unemployment experienced in real-world economies. Nevertheless, it remains an important topic in the study of economics.