Greek philosopher of the 5th century B.C., was born probably at Athens, though Diogenes Laertius (ii. 16) says at Miletus. He was a pupil of Anaxagoras, and is said by Ion of Chios (ap. Diog. Laert. ii. 23) to have been the teacher of Socrates. Some argue that this is probably only an attempt to connect Socrates with the Ionian school; others (e.g. Gomperz, Greek Thinkers ) uphold the story. There is similar difference of opinion as regards the statement that Archelaus formulated certain ethical doctrines. In general, he followed Anaxagoras, but in his cosmology he went back to the earlier Ionians. He postulated primitive Matter, identical with air and mingled with Mind, thus avoiding the dualism of Anaxagoras. Out of this conscious "air," by a process of thickening and thinning, arose cold and warmth, or water and fire, the one passive, the other active. The earth and the heavenly bodies are formed from mud, the product of fire and water, from which springs also man, at first in his lower forms. Man differs from animals by the possession of the moral and artistic faculty. No fragments of Archelaus remain; his doctrines have to be extracted from Diogenes Laertius, Simplicius, Plutarch and Hippolytus.
See Ionian School; for his ethical theories see T. Gomperz, Greek Thinkers (Eng. trans., 1901), vol. i. p. 402.