Ionian school, pre-Socratic group of Greek philosophers of the 6th and 5th cent. B.C.; most of them were born in Ionia. Its members were primarily concerned with the origins of the universe—the forces that shaped it and the materials of which it is composed. Thales, his successor Anaximander, and Anaximenes were all from Miletus. Other prominent members included Anaxagoras, Diogenes of Apollonia, and Archelaus. It is also known as the Milesian school.