Perhaps the single greatest achievement of Greek thought was in philosophy. It was in philosophy that the Greeks most radically broke with supernatural explanations for life and thought and instead sought to establish moral and ethical codes, investigate political theory, and understand human motivations all in terms of the human mind and human capacities. As noted above, the word "philosophy" literally means "love of knowledge," and Greek philosophers did much more than just contemplate the meaning of life; they were often mathematicians, physicists, and literary critics as well as "philosophers" in the sense that that the word is used in the present.
Among the important questions that most Greek philosophers dealt were those concerning politics and ethics. The key question that arose among the early Greek philosophers was whether standards of ethics and political institutions as they existed in Greece - including everything from the polis, democracy, tyranny, Greek standards of behavior, and so on - were somehow dictated by nature or were instead merely social customs that arisen over time. The Classical Age saw the full flowering of Greek engagement with those questions.
Some of the early philosophers of the Greek classical age were the Sophists: traveling teachers who tutored students on all aspects of thought. While they did not represented a truly unified body of thought, the one common sophistic doctrine was that all human beliefs and customs were just habits of a society, that there were no absolute truths, and that it was thus vitally important for an educated man to be able to argue both sides of an issue with equal skill and rhetorical ability. Their focus was on training elite Greeks to be successful – the Greek term for “virtue” was synonymous with “success.” Thus, the sophists were in the business of educating Greeks to be more successful, especially in the law courts and the public assemblies. They did not have a shared philosophical doctrine besides this idea that truth was relative and that the focus in life ought to be on individual achievement.
The men who became the most famous Greek philosophers of all time strongly disagreed with this view. These were a three-person line of teachers and students. Socrates (469 – 399 BCE) taught Plato (428 – 347 BCE), who taught Aristotle (384 – 322 BCE), who went on to be the personal tutor of Alexander the Great for a time. It is one of the most remarkable intellectual lineages in history - three of the greatest thinkers of Greek civilization and one of the greatest military and political leaders, all linked together as teachers and students.
Socrates never wrote anything down; like most of his contemporaries, he believed that writing destroyed the memory and undermined meaning, preferring spoken discourses and memorization. Instead, his beliefs and arguments were recorded by his student Plato, who committed them to prose despite sharing Socrates’ disdain for the written word. Socrates challenged the sophists and insisted that there are essential truths about morality and ethical conduct, but that to arrive at those truths one must be willing to relentlessly question oneself. He took issue with the fact that the sophists were largely unconcerned with ethical behavior, focusing entirely on worldly success; according to Socrates, there were higher truths and meanings to human conduct than mere wealth and political power.
Socrates used what later became known as the “socratic method” to seek out these fixed, unchanging rules of truth and ethics. In the socratic method, the teacher asks a series of questions of the student, forcing the student to examine her own biases and gaps in logic, until finally arriving at a more satisfying and reasonable belief than she started with. In Socrates's case, his questions were meant to lead his interlocutors to arrive at real, stable truths about justice, truth, and virtuous politics. Unlike with the sophists' mastery of rhetoric, the point of the question-and-answer sessions was not to prove that nothing was true, but instead to force one to arrive at truths through the most rigorous application of human reason.
Plato agreed with his teacher that there are essential truths, but he went further: because the senses can be deceived and because our insight is imperfect, only through the most serious contemplation and discussion can we arrive at truth. Truth could only be apprehended with the mind, not with the eye or ear, and it required rigorous discussion and contemplation. To Plato, ideas (which he called "Forms") were more "real" than actual objects. The idea of a table, for instance, is fixed, permanent, and invulnerable, while "real" tables are fragile, flawed, and impermanent. Plato claimed that politics and ethics were like this as well, with the Form of Justice superseding "real" laws and courts, but existing in the intellectual realm as something philosophers ought to contemplate.
In Plato's work The Republic he wrote of an imaginary polis in which political leaders were raised from childhood to become "philosopher-kings," combining practical knowledge with a deep understanding of intellectual concepts. Plato believed that the education of a future leader was of paramount importance, perhaps even more important than that leader's skill in leading armies. Of all his ideas, this concept of a philosopher-king was one of the most influential; various kings, emperors, and generals influenced by Greek philosophy would try to model their rule on Plato's concepts right up to the modern era.
Plato founded a school, the Academy, in Athens, which remained in existence until the early Middle Ages as one of the greatest centers of thought in the world. Philosophers would travel from across the Greek world to learn and debate at the Academy, and it was a mark of tremendous intellectual prestige to study there. It prospered through the entire period of Classical Greece, the Hellenistic Age that followed, and the Roman Empire, only to be disbanded by the Byzantine (eastern Roman) emperor Justinian in the sixth century CE. It was, in short, both one of the most significant and one of the longest-lasting schools in history.
Plato's most gifted student was Aristotle, who founded his own institution of learning, the Lyceum, after he was passed over to lead the Academy following Plato's death. Aristotle broke sharply with his teacher over the essential doctrine of his teaching. Aristotle argued that the senses, while imperfect, are still reliable enough to provide genuine insights into the working of the world, and furthermore that the duty of the philosopher was to try to understand the world in as great detail as possible. One of his major areas of focus was analysis of the real-life politics of the polis; his conclusion was that humans are “political animals” and that it was possible to improve politics through human understanding and invention, not just contemplation.
Aristotle was the ancient world's greatest intellectual overachiever. He single-handedly founded the disciplines of biology, literary criticism, political science, and logical philosophy. He wrote about everything from physics to astronomy and from mathematics to drama. His work was so influential that philosophers continued to believe in the essential validity of his findings well into the period of the Renaissance (thousands of years later) even though many of his scientific conclusions turned out to be factually inaccurate. Despite those inaccuracies, he unquestionably deserves to be remembered as one of the greatest thinkers of all time.