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