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

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.

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

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

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.

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

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.

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.

Leucippus is attributed with the founding of Atomism, the idea that the universe is composed of indivisible elements (atoms) moving through the void.