Long ago, when people believed that the earth was made up of four elements—earth, air, fire, and water—they thought the stars and planets were made up of yet another element. In the Middle Ages, people called this element by its Medieval Latin name, quinta essentia, literally, "fifth essence." Our forebears believed the quinta essentia was essential to all kinds of matter, and if they could somehow isolate it, it would cure all disease. We have since given up on that idea, but we kept quintessence, the offspring of quinta essentia, as a word for the purest essence of a thing. Some modern physicists have given quintessence a new twist—they use it to refer to a form of the dark energy believed to make up almost 70 percent of the energy in the observable universe.