Just as the Greeks developed rituals to honor the gods, they spun myths and epics to explain the origin of the universe. Over time, however, as Greeks encountered other peoples with different beliefs, some of them began to question their old gods and myths, and they sought rational rather than supernatural explanations for natural phenomena. These Greek thinkers, based in Ionia, are called the Pre-Socratics because their rational efforts preceded those of the Athenian. They took individual facts and wove them into general theories that led them to conclude that, despite appearances, the universe is actually simple and subject to natural laws. Although they had little impact on the average Greek of their day, the Pre-Socratics began an intellectual revolution with their idea that nature was predictable, creating what we now call philosophy and science.

Drawing on their observations, the Pre-Socratics speculated about the basic building blocks of the universe. Thales (THAY-leez) (ca. 600 B.C.E.) thought the basic element of the universe was water, and Heraclitus (hehr-uh-KLIGH-tuhs) (ca. 500 B.C.E.) thought it was fire. Democritus (dih-MAW-kruh-tuhs) (ca. 460 B.C.E.) broke this down further and created the atomic theory, the idea that the universe is made up of invisible, indestructible particles. The culmination of Pre-Socratic thought was the theory that four simple substances make up the universe: fire, air, earth, and water.

The stream of thought started by the Pre-Socratics branched into several directions. Hippocrates (hih-PAW-kruh-teez) (ca. 470–400 B.C.E.), who lived on the island of Kos near present-day Turkey, became the most prominent physician and teacher of medicine of his time. He appears to have written several works, and his followers wrote many more. These medical writings became known as the “Hippocratic corpus,” although it is impossible to say who actually wrote any specific work. Hippocrates sought natural explanations for diseases and seems to have advocated letting nature take its course and not intervening too much. Illness was caused not by evil spirits, he asserted, but by physical problems in the body, particularly by imbalances in what he saw as four basic bodily fluids: blood, phlegm, black bile, and yellow bile. In a healthy body these fluids, called humors, were in perfect balance, and the goal of medical treatment of the ill was to help the body bring them back into balance.

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The Sophists (SOFF-ihsts), a group of thinkers in fifth-century-B.C.E. Athens, applied philosophical speculation to politics and language, questioning the beliefs and laws of the polis to understand their origin. They believed that excellence in both politics and language could be taught, and they provided lessons for the young men of Athens who wished to learn how to persuade others in the often-tumultuous Athenian democracy. Their later opponents criticized them for charging fees and also accused them of using rhetoric to deceive people instead of presenting the truth. (Today the word sophist is usually used in this sense, describing someone who deceives people with clever-sounding but false arguments.)

Socrates (SOK-ruh-teez) (ca. 469–399 B.C.E.), whose ideas are known only through the works of others, also applied philosophy to politics and to people. He seemed to many Athenians to be a Sophist because he also questioned Athenian traditions, although he never charged fees. His approach when exploring ethical issues and defining concepts was to start with a general topic or problem and to narrow the matter to its essentials. He did so by continuously questioning participants in a discussion or argument through which they developed critical thinking skills, a process known as the Socratic method. Because he posed questions rather than giving answers, it is difficult to say exactly what Socrates thought about many things, although he does seem to have felt that through knowledge people could approach the supreme good and thus find happiness. He clearly thought that Athenian leaders were motivated more by greed and opportunism than by a desire for justice in the war with Sparta, and he criticized Athenian democracy openly.

Socrates was viewed with suspicion by many because he challenged the traditional beliefs and values of Athens. The playwright Aristophanes satirized him and his followers in the riotously funny The Clouds, performed around 420 B.C.E. Twenty years later, after Athens’s disastrous defeat at the hands of Sparta in the Peloponnesian War, Socrates came into serious conflict with the government. Charges were brought against him for corrupting the youth of the city, and for impiety, that is, for not believing in the gods honored in the city. Thus he was essentially charged with being unpatriotic because he criticized the traditions of the city and the decisions of government leaders. He was tried and imprisoned, and though he had several opportunities to escape, in 399 B.C.E. he drank the poison ordered as his method of execution and died.

Most of what we know about Socrates, including the details of his trial and death, comes from his student Plato (427–347 B.C.E.), who wrote dialogues in which Socrates asks questions and who also founded the Academy, a school dedicated to philosophy. Plato developed the theory that there are two worlds: the impermanent, changing world that we know through our senses, and the eternal, unchanging realm of “forms” that constitute the essence of true reality. According to Plato, true knowledge and the possibility of living a virtuous life come from contemplating ideal forms — what later came to be called Platonic ideals — not from observing the visible world. Thus if you want to understand justice, asserted Plato, you should think about what would make perfect justice, not study the imperfect examples of justice around you. Plato believed that the ideal polis could exist only when its citizens were well educated. From education came the possibility of determining all of the virtues of life and combining them into a system that would lead to an intelligent, moral, and ethical life.

Plato’s student Aristotle (384–322 B.C.E.) also thought that true knowledge was possible, but he believed that such knowledge came from observation of the world, analysis of natural phenomena, and logical reasoning, not contemplation. Aristotle thought that everything had a purpose, so that to know something, one also had to know its function. Excellence — arête in Greek — meant performing one’s function to the best of one’s ability, whether one was a horse or a person. To the qualities of courage and strength that Homer had seen as essential to arête in people (see “Homer, Hesiod, and the Epic”), Aristotle added justice, generosity, temperance, and other moral virtues. The range of Aristotle’s thought is staggering. His interests embraced logic, ethics, natural science, physics, politics, poetry, and art. He studied the heavens as well as the earth and judged the earth to be the center of the universe, with the stars and planets revolving around it.

Plato’s idealism profoundly shaped Western philosophy, but Aristotle came to have an even wider influence; for many centuries in Europe, the authority of Aristotle’s ideas was second only to the Bible’s. His works — which are actually a combination of his lecture notes and those of his students, copied and recopied many times — were used as the ultimate proof that something was true, even if closer observation of the phenomenon indicated that it was not. Thus, ironically, Aristotle’s authority was sometimes invoked in a way that contradicted his own ideas. Despite these limitations, the broader examination of the universe and the place of humans in it that Socrates, Plato, and Aristotle engaged in is widely regarded as Greece’s most important intellectual legacy.