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 years tantalized our greatest thinkers, from Montaigne to Nietzsche, Heidegger to Jung. Now, acclaimed poet Brooks Haxton presents a powerful free-verse translation of all 130 surviving fragments of the teachings of Heraclitus, with the ancient Greek originals beautifully reproduced ''en face.''