Fragments of wisdom from the ancient world In the sixth century b.c.-twenty-five hundred years before Einstein--Heraclitus of Ephesus declared that energy is the essence of matter, that everything becomes energy in flux, in relativity. His great book, On Nature , the world's first coherent philosophical treatise and touchstone for Plato, Aristotle, and Marcus Aurelius, has long been lost to history--but its surviving fragments have for thousands of...