ARCHELAUS, a celebrated Greek philosopher, born probably at Miletus about 470 B.C. He succeeded his master Anaxagoras in the school which he had founded at Lampsacus, from whence he afterwards removed to Athens. He was the last representative of the Ionian school, between which and the more practical philosophy that followed, his teaching formed a link of transition. His speculations were chiefly, but not exclusively physical, on which account he received the name of the Physician (ὁ φυσικός), in contradistinction to his contemporaries the Sophists, and his illustrious disciple Socrates. His opinions were, for the most part reproductions of the ideas of Anaxagoras. The following are some of the peculiar physical doctrines ascribed to him. He taught that there was a double principle of all things, namely, the expansion and condensation of the air, which he regarded as infinite. Heat, according to him, was in continual motion; cold was ever at rest. The earth, which was placed in the midst of the universe, had no motion. It originally resembled a wet marsh, but was afterwards dried up; and its figure, he said, was not flat but spherical, which he deduced from the fact that the sun does not become visible at the same time in all places. The sea he regarded as the overflowing of the waters contained within the earth. Animals were produced from the heat of the earth, and even men were formed in the same manner. All animals have a soul, which was born with them, but the capacities of which vary according to the structure of the organs of the body in which it resides. In morals he showed his approximation to the Sophists, by teaching that right and wrong are founded not in nature but on custom.