So did plagues kill off part of the work force? Did termites eat part of the capital stock? Did technology retrogress? No, no. no. On the last point, has anyone noticed that the iPhone was introduced in 2007, and that the whole smartphone/tablet revolution has more or less coincided with a period of terrible economic performance?
So what did happen? Keynes offered an answer: it is, in fact, possible for economies to suffer from an overall lack of demand. Other people had said things along these lines, but Keynesian economics put it front and center.
This really was an intellectual revolution; indeed, while I’m generally against scientific pretensions, it amounted to a scientific revolution, something like plate tectonics in geology. Suddenly the seemingly inexplicable — what elevates mountain ranges? what explains periods of economic retrogression? — became comprehensible.