Anaximenes was a 6th century B.C. Pre-Socratic philosopher and member of the Milesian School who is credited with the theory of change.
From the Internet Encyclopedia of Philosophy. Anaximenes was a contemporary and pupil of Anaximander who died around 502 B.C. He developed the theory of rarefaction and condensation.
Greek Materialists: Thales, Anaximander and Anaximenes
They looked at the underlying substance of the universe which they believed was a material substance. For Anaximenes it was air; for Thales, water; for Heraclitus, fire; for Anaximander; an indefinable "apeiron."
Major Greek and Roman philosophers in chronological order.