Keynes was the great economist of the 20th century. His 1936 book "The General Theory of Employment, Interest, and Money" constituted an intellectual revolution. Contrary to the prevailing doctrines of the day, he argued that markets left alone could remain mired in extended periods of deep unemployment. Even if there were self-correcting forces bringing the economy back to full employment, they worked too slowly to prevent significant economic hardship.
He explained why monetary policy—favoured by many conservative economists when intervention was deemed necessary—would be ineffective in a deep downturn. Most importantly, he provided a solution: government spending could stimulate demand and lift the economy out of the mire.
Keynes was no left-wing radical; he was not overly concerned with inequality. He believed in the market economy and believed that his proposed intervention—not a revolution, but a minor "fix"—would save capitalism from itself. As he quipped, in the long run we're all dead.
His ideas were tested in practice by Franklin Roosevelt's New Deal and later adopted as the cornerstone of President John F. Kennedy's economic framework, under the influence of Keynesian economists including John Kenneth Galbraith, Robert Solow and Paul Samuelson. Throughout the 1970s, with America facing inflation largely caused by unprecedented increases in oil prices, the right claimed Keynes was passé. Yet during the Great Recession and the covid-19 pandemic, Keynesian interventions proved enormously effective.
Sturgeon's Law: 90% of everything is crud.