[C]an we say—outside a chemical reaction—­what human energy is? For centuries, we have been trying to answer that question. It was of particular interest to late-nineteenth and early-twentieth-century philosophers. Schopenhauer wrote about man’s “will to live,” Freud of the libido being energy’s vital source, while in 1907, in his book Creative Evolution, the French philosopher Henri Bergson identified an élan vital that impelled consciousness and evolution. This notion also corresponds to the German word Lebensdrang, which translates as “life-urge.” Most languages have their own terms for it: Scottish Gaelic has two, lùth (“energy”) and beòthachd (“vigor”); Mandarin Chinese three, lìliàng (“strength”), néngyuán (“power”), and jīnglì (energies in the plural); and Hindi at least four, covering energy systems, consumption, sources, and supply.