There Arose 3 Broad Conclusions based on Say's Law: (1) Goods Buy Goods, (2) Demand Is Constituted by Supply, and (3) There Is No Such Thing As a General Glut

From this principle, there arose three broad conclusions that were accepted by the mainstream of the economics profession through until 1936. These are the conclusions based on an understanding of Say's Law:

1. 'Goods buy goods': to buy, one first has to produce goods of one's own, sell these goods for money and then use the money received to buy the goods produced by others; thus it is the production of one's own goods that leads to the ability to purchase someone else's, even though money is used as the medium of exchange
2. 'Demand is constituted by supply': in an exchange economy one cannot buy unless they have first supplied, since unless someone supplies they have no money with which to demand
3. 'There is no such thing as a general glut': it is impossible for an economy to produce so much output that there would not be enough buyers for what has been produced, so long as what has been produced is what people want to buy; therefore demand deficiency across an entire economy can never be a realistic explanation for recession.

It was the reversal of this last principle in particular, following the publication in 1936 of The General Theory of Employment, Interest and Money by the economist John Maynard Keynes--the most influential text on economics written during the whole of the twentieth century--that led to what became known as the 'Keynesian Revolution'.

--Steven Kates, Free Market Economics: An Introduction for the General Reader, 3rd ed. (Cheltenham, UK: Edward Elgar Publishing, 2017), Kobo e-book.