Cratylus (ancient Greek: Κρατύλος, Kratylos) was an ancient Athenian philosopher from late 5th century BC, mostly known through his portrayal in Plato's ialogue Cratylus. Little is known of Cratylus or his mentor Heraclitus (of Ephesus, Asia Minor). According to Cratylus at 402a, Heraclitus proclaimed that one cannot step twice into the same stream. According to Aristotle (Metaphysics, 4.5 1010a10-15), his disciple Cratylus went a step further to proclaim that it cannot even be done once. Such was his thorough-going skepticism.