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

Plato's theory of Forms, also known as the Theory of Ideas, posits that there is a realm of perfect and eternal Forms that exist independently of the physical world.

Aristotle was the tutor of Alexander the Great, who went on to become one of the most influential military leaders in history.

Aristotle's Nicomachean Ethics is a treatise on moral philosophy that explores the nature of virtue and happiness.

Pyrrho, the founder of Pyrrhonism, is known for his skeptical approach to knowledge, arguing that all beliefs are uncertain and that one should suspend judgment.

Epicurus founded the philosophical school known as Epicureanism, which emphasized the pursuit of happiness through moderation and the avoidance of pain.

Aristotle's theory of the four causes explains that every event or object has four causes: material, formal, efficient, and final.

Zeno of Citium founded the philosophical school known as Stoicism, which emphasized the importance of virtue, reason, and living in accordance with nature.

John Locke, a British philosopher, is known for his theory of the social contract, which posits that individuals give up some of their freedoms in exchange for the protection and benefits provided by society.

Diogenes of Sinope founded the philosophical school known as Cynicism, which emphasized living a simple and virtuous life, rejecting material possessions and social conventions.

Immanuel Kant, a German philosopher, is known for his theory of the categorical imperative, which states that one should act only according to the maxim by which one can, at the same time, will that it should become a universal law.

Epictetus, a Greek philosopher, was a prominent figure in the Stoic school, emphasizing the importance of virtue, reason, and living in accordance with nature.

Charles Sanders Peirce, an American philosopher, is known for his theory of the three modes of being: firstness, secondness, and thirdness.

Pyrrho of Elis founded the philosophical school known as Skepticism, which emphasized the uncertainty of knowledge and the importance of suspending judgment.

Friedrich Nietzsche, a German philosopher, is known for his theory of the Übermensch or Superman, which posits the existence of a superior being that transcends conventional morality and values.