School of Greek philosophers of the 6th–5th century BC, including Anaximander , Anaximenes , Heracleitus , Anaxagoras , Diogenes of Apollonia, Archelaus, and Hippon.
Though Ionia was the original center of their activity, they differed so greatly from one another in their conclusions that they cannot truly be said to represent a specific school of philosophy, but their common concern to explain phenomena in terms of matter or physical forces distinguished them from later thinkers.