Heraclitus believed that everything in the universe is in a constant state of flux, or change.

Parmenides argued that reality is a single, unchanging substance, and that change is an illusion.

Anaximander believed that the primary substance from which all things are made is Apeiron, an infinite and undefined substance.

Anaximenes believed that the primary substance from which all things are made is air.

Democritus and Leucippus are credited with developing the theory of atomism, which states that all matter is made up of tiny, indivisible particles called atoms.

Pythagoras believed that numbers were the fundamental principle of the universe, and that everything could be explained in terms of mathematical ratios.

Empedocles believed that all matter was made up of four elements: earth, air, fire, and water.

Anaxagoras believed that the primary substance from which all things are made is Nous, or mind.

Leucippus and Democritus proposed the theory of the void, which states that there is an empty space between atoms.

Pythagoras believed that music and mathematics were closely related, and that the harmonies of music could be expressed in mathematical terms.

Pythagoras is credited with discovering the golden mean, a mathematical ratio that is found in many natural and man-made objects.

Empedocles believed that change in the universe was caused by the interaction of two opposing forces: Love and Strife.

Aristotle developed the theory of the four causes, which states that everything has four causes: material, formal, efficient, and final.

Anaxagoras believed that the primary substance from which all things are made is Nous, or mind.

Leucippus and Democritus proposed the theory of the monad, which states that all matter is made up of tiny, indivisible particles called monads.