In ancient Greek philosophy, an infinite, boundless, and indeterminate substance or principle that was thought to be the fundamental source of all existence.
From Greek 'apeiron' (unlimited, boundless) from 'a-' (not) + 'peirar' (limit, end). The term was coined by the pre-Socratic philosopher Anaximander around 600 BCE to describe the primal element underlying reality.
Anaximander proposed 'apeiron' as the ultimate building block of the universe—not water like Thales suggested, but literally infinite and formless, making it one of the earliest attempts at theoretical physics and the ancestor of modern concepts like dark matter.
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