Keynes revolutionized modern economics by stating that aggregate demand is what controls the economy, not the free market. He is considered to be one of the most influential economists of the 20th Century as he invented modern macroeconomics. Liberal Innovator.
 

"Practical men who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist. Madmen in authority, who hear voices in the air, are distilling their frenzy from some academic scribbler of a few years back."

— John Maynard Keynes