Thales of Miletus proposed that all matter is composed of four basic elements: earth, air, fire, and water.

Heraclitus argued that everything is in a state of constant flux and change, and that the only constant is change itself.

Democritus and Leucippus developed the atomic theory, which proposed that all matter is composed of tiny, indivisible particles called atoms.

Heraclitus believed that the universe is governed by a single, unchanging principle called the Logos, which is responsible for the order and harmony of the cosmos.

Protagoras argued that all knowledge is based on sense perception, and that there is no objective truth.

Parmenides is considered the founder of the Eleatic school of philosophy, which argued that reality is one, unchanging, and indivisible.

Socrates developed a theory of knowledge based on rational thought and argumentation, and he is considered the founder of Western philosophy.

Zeno of Elea is known for his paradoxes that challenge the notion of change and motion, such as the paradox of the arrow and the paradox of Achilles and the tortoise.

Anaxagoras argued that the universe is composed of an infinite number of seeds or elements, which are the basic building blocks of all matter.

Pythagoras is considered the founder of the Pythagorean school of philosophy, which emphasized the importance of mathematics and music in understanding the universe.

Plato developed a theory of knowledge based on the idea of recollection, which argues that knowledge is innate and can be accessed through rational thought and contemplation.

Aristotle developed a theory of the four causes, which explains the different ways in which something can be caused or explained.

Empedocles argued that the universe is composed of two opposing forces, love and strife, which interact to create the diversity of the world.

Antisthenes is considered the founder of the Cynic school of philosophy, which emphasized the importance of virtue and self-sufficiency.

Anaximander argued that the universe is composed of an infinite number of worlds, each of which is surrounded by a sphere of air.