John Maynard Keynes once observed that dating from “say, to two thousand years before Christ—down to the beginning of the 18th century, there was no very great change in the standard of life of the average man living in the civilised centres of the earth. Ups and downs certainly. Visitations of plague, famine, and war. Golden intervals. But no progressive, violent change.” At best, he calculated, the average standard of living had no more than doubled in the previous four millennia, essentially because, when that epoch began, we already knew about fire, banking, the sail, the plow, mathematics; we learned little new that would have accelerated economic growth; and throughout that stretch the planet mostly ran on the muscles of people and animals, supplemented by the power of wind and water. Then, in the eighteenth and nineteenth centuries, we started to harness the combustion of coal, gas, and oil, and everything changed. That’s because a barrel of oil contains 5.8 million British thermal units’ worth of energy.
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