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Considered the first Greek philosopher. He theorized that water is the underlying substance of everything, purposed that natural phenomena have natural explanations, and attempted to predict solar eclipses.

Introduced the concept of the 'apeiron' (the unlimited), believed to be the origin of all things, and speculated on the development of life from the sea and the evolution of species.

Proposed that air is the primary substance of reality and that other forms of matter are formed through condensation and rarefaction.

Developed early mathematical concepts, believed that numbers are the true essence of reality, and established a religious/mystical school that held that the soul is immortal and engages in a cycle of rebirth.

Famous for his doctrine that everything is in constant flux and his belief in the unity of opposites. He also held that fire is the primary substance of reality and a symbol of change.

Offered a metaphysical claim that reality is one indivisible and unchangeable whole, arguing that change and plurality are illusions. He created the deductive method of argument.

Introduced the theory of the four classical elements (earth, water, air, fire) as roots of all matter and combined this with the forces of love and strife to account for the combination and separation of elements.

Developed the notion of 'nous' (mind or intellect) as a cosmic principle that imposes order. He also offered explanations for celestial phenomena and is credited with providing a naturalistic explanation of the sun and moon.

Best known for formulating an atomic theory of the universe. He theorized that all matter is composed of indivisible atoms moving through the void (empty space).

Credited with originating the atomic theory alongside his student Democritus. Although less is known about Leucippus, he laid the foundations for the concept of atoms and void.

Criticized the anthropomorphic gods of Greek mythology and posited a monotheistic conception of god. He used fossils and geology as evidence for the theory that Earth was once covered by water.