(c. 469 BC-399 BC) We have no writings from his own hand, and know about him largely from the dialogues of his student Plato.
Proclaiming his own ignorance of all things, he went around Athens engaging in question-and-answer sessions to search for truths or draw out contradictions (the "Socratic method").
The Athenian state disapproved of his conduct, and he was put on trial for corrupting the city's youth, which led to his death by drinking hemlock. His trial, imprisonment, and death are recounted in Plato's Apology, Crito, and Phaedo, respectively.
(c. 429 BC-347 BC) His Socratic dialogues are our main source both for Socrates's philosophy and his own; he often put his own thoughts in Socrates' mouth.
His dialogues include the Republic (about justice and the ideal city-state), the Symposium (about the nature of love), and the Meno (about whether virtue can be taught). He believed in a world of "forms"—or ideal versions of real things that lie beyond the human senses—which he discussed in such works as the Phaedo.
This author of The Republic founded a school called the Academy, from which we get the common word.
(c. 384 BC-322 BC) A student of Plato; and a tutor to Alexander the Great. Many of his works come to us in the form of lectures he gave at his school, known as the Lyceum.
His philosophical output includes the Nicomachean Ethics, which argues that virtues consist in a "golden mean" between two extremes; the Physics, which describes motion and change in terms of "four causes" that make a given thing what it is; and the Metaphysics, which describes the structure of reality.
His Poetics discusses the types of drama and considers an effect of tragedies known as catharsis, or the purging of bad feelings.
(or Kong Fu Zi) (6th century BC) A pivotal thinker from China's Spring and Autumn period, his views on proper conduct and filial piety still influence China to this day.
Many sayings attributed to him were compiled by his disciples following his death in a text known as the Analects.
He put much importance on ren, the inner state which allows one to behave compassionately toward others, and on a concept called li, which can help individuals attain ren.
(also given as Lao Tse or Laozi) (dates unknown, 6th century BC) A quasi-mythical thinker of the Taoist tradition, to whom the pivotal Tao te Ching is attributed.
Concepts associated with him include that of the Tao, or "the way," and wu wei, or a life of non-action in accordance with the Tao. In later centuries, he was accorded godlike status as one of the Three Pure Ones of Taoism, and is frequently depicted as an old man with a donkey.
He supposedly said "The journey of a thousand miles begins with a single step."
(c. 410s BC-323 BC) A student of Antisthenes, who founded the ancient school of philosophy known as Cynicism. (The term "cynic" comes from the Greek for "dog-like," and is thought to have originated as an insult to the school's members.) The Cynics rejected conventional social norms in search of a truly virtuous life.
Something of an eccentric—according to legend, he lived in a tub or a barrel on the street, and wandered Athens holding a lamp in his futile search for an honest man.
(341 BC-270 BC) His namesake school, Epicureanism, believed that pleasure was the highest (or only) good, and that the absence of pain (aponia) was the highest pleasure. They also believed that human happiness consisted of a kind of tranquillity known as ataraxia. Critics of Epicureanism accused his school of promoting hedonism and making selfishness into a good, though Epicureans did not believe themselves to be hedonists.
Zeno of Elea
(c. 490 BC-430 BC) A student of Parmenides, who founded the Eleatic school in a Greek colony of the Italian peninsula. He is most famous today for "Zeno's paradoxes," the best-known of which involve an arrow in flight and a race between Achilles and a tortoise.
Zeno of Elea
His paradoxes purport to show that physical movement is impossible, since any attempt to travel a distance must be preceded by moving half that distance, which must be preceded by moving half of half that distance, and so on. (He is not to be confused with Zeno of Citium, who founded the philosophical school of Stoicism two centuries later.)
(c. 620 BC-546 BC) A pre-Socratic thinker from the Greek colony of Miletus who many consider to be the "first philosopher." Rejecting mythical explanations of the universe's nature, he believed that the first principle of all existence, the natural element from which all things emerged, was water.
He was also a civil engineer and mathematician, and is credited with discovering that any triangle whose hypotenuse is the diameter of a circle must be a right triangle.
He is sometimes thought of as the founder of a "Milesian school" of philosophy, whose other members include Anaximander and Anaximenes.
(106 BC-43 BC) Though he is better remembered today for his role in the political life of the Roman Republic, he was also a significant philosopher.
He described the ideal state in such dialogues as On the Republic and On the Laws, while he discussed Epicurean and Stoic views on religion in On the Nature of the Gods.
Through the Middle Ages and Renaissance, he was considered one of the most important of ancient philosophers. Indeed, Saint Augustine asserted that he turned to philosophy as a result of reading a now-lost work by him known as the Hortensius.