Virtually nothing is known of the life of Heraclitus and of his book On Nature only a few rather obscure fragments survive. His doctrines contrast with those of his near contemporary Parmenides for whom, on purely logical grounds, change of any kind was totally impossible. For Heraclitus, everything is continually in a state of change, hence his characteristic aphorism: “We cannot step twice into the same river,” and his selection of fire as the fundamental form of matter. The mechanism behind such unremitting change was the constant tension or ‘strife’ between contraries or opposites.