Heraclitus of Ephesus (Greek Ηρακλειτος Herakleitos) (about 535 - 475 BC), known as 'The Obscure,' was a pre-Socratic Greek philosopher. He disagreed with Thales, Anaximander, and Pythagoras about the nature of the ultimate substance and claimed instead that everything is derived from the Greek classical element fire, rather than from air, water, or earth. This led to the belief that change is real, and stability illusory. For Heraclitus everything is "in flux", as exemplified in his famous aphorism "Panta Rhei":
Παντα ρει και ουδεν μενει
Everything flows, nothing stands still
He is famous for saying: "No man can cross the same river twice, because neither the man nor the river are the same." The idea of the logos is also credited to him, as he believes everything originates out of the logos. Further, Heraclitus said "I am as I am not".
Heraclitus' view that an explanation of change was foundational to any theory of nature was strongly opposed by Parmenides, who argued that change is an illusion and that everything is fundamentally static.
Only fragments of Heraclitus' writings have been found. He appears to have taught by means of small, oracular aphorisms meant to encourage thinking based on natural law and reason. The brevity and elliptical logic of his aphorisms earned Heraclitus the epithet 'Obscure'.