John Maynard Keynes
John Maynard Keynes revolutionized economic thought during the Great Depression by demonstrating that free markets do not automatically correct themselves.
Total economic output and employment are determined by aggregate demand rather than supply.
During economic downturns, governments must intervene with active fiscal policy to stimulate demand and restore growth.
The interest rate is determined by the supply of money and the public's desire to hold cash rather than just the supply of loanable funds.