Heraclitus (sometimes Heracleitus) was a pre-Socratic Greek philosopher whose obscure brand of metaphysics has been boiled down to the tidy maxim "you can never step in the same river twice." He is known for proposing that the universe is a balance of opposing forces constantly in flux, and for calling the basic universal constituent "fire." What little is known about Heraclitus comes from later writers, including Plato and Aristotle, who characterized his philosophy as contradictory. According to early biographers, Heraclitus was melancholic and cryptic, earning him the nicknames "The Weeping Philosopher' and "The Riddler." One of the earliest metaphysicians, he is considered an influence on modern ideas such as relativity and process theology.