|Heraclitus (540 BC - 480 BC)
Heraclitus of Ephesus , 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".
He is famous for saying: "No man can cross the same river twice, because neither the man nor the river are the same."
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 elliptic logic of his aphorisms earned Heraclitus the epithet 'Obscure'.