Heraclitus of Ephesus (about 535 - 475 B.C.), presocratic Greek philosopher, disagreed with Thales, Anaximander, and Pythagoras about the nature of the ultimate substance. He claimed instead that everything is derived from fire, rather than air, water, or earth. This lead 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.