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Anaximander theorized the existence of ‘apeiron’ (the boundless or infinite), from which all things emerge and to which they return—a concept that suggests a form of eternal motion and transformation.

Empedocles introduced the theory of the four roots—earth, air, fire, and water—as elements that combine and separate under the forces of love and strife.

Anaximenes postulated that air is the primary substance of everything, with the processes of rarefaction and condensation explaining variation in material existence.

Anaxagoras introduced the concept of Nous (Mind or Intellect) as the original force of order that separated and arranged the chaotic infinite particles into the cosmos.

Democritus, a proponent of Atomism alongside Leucippus, held that atoms are varied and infinite in shape and size and that their motion through the void is the cause of all natural phenomena.

Parmenides argued for the idea of a singular, unchanging reality ('the One') that is timeless, uniform, and unchanging, contrasting sharply with Heraclitus.

Thales believed that the fundamental principle of all things (archê) is water, implying that everything can be reduced to this one element.

Heraclitus believed in an ever-changing universe governed by a law he called the Logos, with fire as the primary substance symbolizing constant change and flux.

Pythagoras identified number as the fundamental principle, positing a cosmos that is structured by harmony and mathematical relations.