Socrates is widely regarded as the father of Western philosophy due to his profound influence on subsequent philosophers and his contributions to ethical thought.

René Descartes is credited with the famous statement "Cogito, ergo sum" (I think, therefore I am), which is a fundamental principle of his philosophy.

Plato's Allegory of the Cave is a parable that explores the relationship between reality and our perception of it.

Aristotle's theory of the four causes (material, formal, efficient, and final) is a fundamental concept in his philosophy.

Stoicism is a Hellenistic philosophy founded by Zeno of Citium that emphasizes virtue, reason, and acceptance of the natural order.

Epicurus is known for his hedonistic philosophy, which advocates for the pursuit of pleasure as the highest good.

Skepticism is a philosophical school founded by Pyrrho of Elis that questions the possibility of certain knowledge.

Diogenes of Sinope is known for his philosophy of Cynicism, which advocates for a simple and virtuous life.

Platonism is a philosophical school founded by Plato that emphasizes the existence of a transcendent realm of Forms.

Plato's theory of the Forms is a fundamental concept in his philosophy, which posits that there are perfect and unchanging entities beyond the physical world.

Pythagoreanism is a philosophical school founded by Pythagoras that emphasizes the importance of mathematics and harmony in the universe.

Pythagoras is known for his theory of the transmigration of souls, which posits that the soul passes through a cycle of rebirth.

Epicureanism is a philosophical school founded by Epicurus that emphasizes the pursuit of pleasure and the avoidance of pain.

Empedocles is known for his theory of the four elements (earth, air, fire, and water) as the fundamental constituents of matter.

Cyrenaicism is a philosophical school founded by Aristippus of Cyrene that emphasizes the pursuit of pleasure as the highest good.