The central argument of The General Theory is that the level of employment is determined not by the price of labour, as in classical economics, but by the spending of money (aggregate demand). Keynes argues that it is wrong to assume that competitive markets will, in the long run, deliver full employment or that full employment is the natural, self-righting, equilibrium state of a monetary economy. On the contrary, underemployment and underinvestment are likely to be the natural state unless active measures are taken.

John Maynard Keynes, 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 is widely considered to be one of the most influential economists of the 20th century and the founder of modern macroeconomics theory. His ideas are the basis for the school of thought known as Keynesian economics, and its various offshoots. Keynes' magnum opus, The General Theory of Employment, Interest and Money, was published in 1936.