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===Philosophical models===
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From the 6th century BCE, the [[pre-Socratic philosophy|pre-Socratic Greek philosophers]] developed the earliest known philosophical models of the Universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the apparently different materials of the world (wood, metal, etc.) are all different forms of a single material, the [[arche]]. The first to do so was [[Thales]], who called this material [[Water (classical element)|Water]]. Following him, [[Anaximenes]] called it [[Air (classical element)|Air]], and posited that there must be attractive and repulsive [[force]]s that cause the arche to condense or dissociate into different forms. [[Empedocles]] proposed that multiple fundamental materials were necessary to explain the diversity of the universe, and proposed that all four classical elements (Earth, Air, Fire and Water) existed, albeit in different combinations and forms. This four-element theory was adopted by many of the subsequent philosophers. Some philosophers before Empedocles advocated less material things for the arche; [[Heraclitus]] argued for a [[Logos]], [[Pythagoras]] believed that all things were composed of [[number]]s, whereas Thales' student, [[Anaximander]], proposed that everything was composed of a chaotic substance known as [[Apeiron (cosmology)|apeiron]], roughly corresponding to the modern concept of a [[quantum foam]]. Various modifications of the apeiron theory were proposed, most notably that of [[Anaxagoras]], which proposed that the various matter in the world was spun off from a rapidly rotating apeiron, set in motion by the principle of [[Nous]] (Mind). Still other philosophers — most notably [[Leucippus]] and Democritus — proposed that the Universe was composed of indivisible [[atom]]s moving through empty space, a [[vacuum]]; [[Aristotle]] opposed this view ("Nature abhors a vacuum") on the grounds that [[Drag (physics)|resistance to motion]] increases with [[density]]; hence, empty space should offer no resistance to motion, leading to the possibility of infinite [[speed]].
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