Apeiron (ἄπειρον) — “The unlimited” or “the indefinite.” The arché (principle) proposed by Anaximander of Miletus (~610–546 BCE) as the substrate of all things. Unlike Thales (water) or Anaximenes (air), Anaximander argued that no determinate element can be the principle of all things — the principle must itself be indeterminate, eternal, and in perpetual motion, from which opposites (hot/cold, dry/moist) emerge to generate the world.
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