Anaximander ənăkˌsĭmănˈdər [key], c.611–c.547 b.c., Greek philosopher, b. Miletus; pupil of Thales. He made the first attempt to offer a detailed explanation of all aspects of nature. Anaximander argued that since there are so many different sorts of things, they must all have originated from something less differentiated than water, and this primary source, the boundless or the indefinite (apeiron), had always existed, filled all space, and, by its constant motion, separated opposites out from itself, e.g., hot and cold, moist and dry. These opposites interact by encroaching on one another and thus repay one another's “injustice.” The result is a plurality of worlds that successively decay and return to the indefinite. The notion of the indefinite and its processes prefigured the later conception of the indestructibility of matter. Anaximander also had a theory of the relation of earth to the heavenly bodies, important in the history of astronomy. His view that man achieved his physical state by adaptation to environment, that life had evolved from moisture, and that man developed from fish, anticipates the theory of evolution.

See studies by P. Selegman (1974), C. H. Kahn (3d ed. 1994), and C. Rovelli (tr. 2011).

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