apeiron

The apeiron (Greek ἄπειρον) means "the unlimited" or "the infinite." It is a term used by the pre-Socratic philosopher Anaximander (c. 610–546 BCE) to designate the arche, the first principle from which all things arise and to which they return.

In Anaximander's system, the apeiron is not a concrete substance such as water or air but a timeless, boundless principle. It is eternal and ageless, yet it generates the order of the world through process rather than by imitating any particular material. The differentiation of opposites—hot and cold, wet and dry—emerges from the apeiron in a cyclical, balancing play; from this differentiation the diverse phenomena of the visible world come into being, exist for a time, and ultimately perish back into the indefinite.

The apeiron thus functions as the source of the cosmos and the principle governing change, structure, and the regularities of nature. It explains why order arises from apparent chaos and why finite things are variable yet governed by enduring patterns. The concept also served as an early attempt to articulate a universal, impersonal cause rather than mythic forces.

Reception: Later philosophers, notably Aristotle, discussed and critiqued the apeiron as a philosophical model of the infinite or indefinite. Surviving evidence for the apeiron is indirect, known primarily through fragments and later summaries.