“It seems an extraordinary imbecility that this wonderful outburst of productive energy should be the prelude to impoverishment and depression,” Keynes wrote in 1931, after the boom years of the 1920s had given way to the Great Depression. “I find the explanation of the current business losses, of the reduction in output, and of the unemployment which necessarily ensues on this not in the high level of investment which was proceeding up to the spring of 1929, but in the subsequent cessation of this investment.”