Learn about this topic in these articles:

In Anaximander
…that everything originated from the apeiron (the “infinite,” “unlimited,” or “indefinite”), rather than from a particular element, such as water (as Thales had held). Anaximander postulated eternal motion, along with the apeiron, as the originating cause of the world. This (probably rotary) motion caused opposites, such as hot and cold,…
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In Eleaticism: The Eleatic school vis-à-vis rival movements
Though Anaximander’s basic principle, the apeiron (“boundless”), was duly abstract and not a part of the world itself (as were water and air), his philosophy depended, nonetheless, upon the world’s contrast with the infinite apeiron, from which all things come and to which they return “in accordance with the ordinance…
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In Western philosophy: Monistic cosmologies
…it developed out of the apeiron (“unlimited”), something both infinite and indefinite (without distinguishable qualities). Within this apeiron something arose to produce the opposites of hot and cold. These at once began to struggle with each other and produced the cosmos. The cold (and wet) partly dried up (becoming solid…
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