What is Keynesian law and what is the socio-economic background put forward by Keynesian law?

When the total social demand changes, it will only cause changes in output and income, make supply and demand equal, and will not cause price changes. This is sometimes called Keynesian law in western economics. When Engels wrote the General Theory of Employment, Interest and Money, he was faced with the Great Depression of 1929- 1933, in which a large number of workers were unemployed and a large number of resources were idle. In this case, when the total social demand increases, only idle resources are utilized and the output increases, but the price of resources will not rise, so the cost and price of products remain basically unchanged. This so-called Keynesian law is considered to be suitable for short-term analysis, that is, how to determine income and employment in the short term.