John Maynard Keynes, 1st Baron Keynes ( KAYNZ; 5 June 1883 – 21 April 1946), was a British economist whose ideas fundamentally changed the theory and practice of macroeconomics and the economic policies of governments. He built on and greatly refined earlier work on the causes of business cycles, and was one of the most influential economists of the 20th century. Widely considered the founder of modern macroeconomics, his ideas are the basis for the school of thought known as Keynesian economics, and its various offshoots.During the Great Depression of the 1930s, Keynes spearheaded a revolution in economic thinking, challenging the ideas of neoclassical economics that held that free markets would, in the short to medium term, automatically provide full employment, as long as workers were flexible in their wage demands. He argued that aggregate demand (total spending in the economy) determined the overall level of economic activity, and that inadequate aggregate demand could lead to prolonged periods of high unemployment. Keynes advocated the use of fiscal and monetary policies to mitigate the adverse effects of economic recessions and depressions. He detailed these ideas in his magnum opus, The General Theory of Employment, Interest and Money, published in 1936. In the mid to late-1930s, leading Western economies adopted Keynes's policy recommendations. Almost all capitalist governments had done so by the end of the two decades following Keynes's death in 1946.