Just as the Greeks developed rituals to honor gods, they spun myths and epics to explain the origins of the universe. Over time, however, some Greeks 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 better-known Socrates. Taking individual facts, they wove them into general theories that led them to conclude that, despite appearances, the universe is actually simple and subject to natural laws.

Drawing on their observations, the Pre-Socratics speculated about the basic building blocks of the universe, and most decided that all things were made of four simple substances: fire, air, earth, and water. Democritus (dih-MAW-kruh-tuhs) (ca. 460 B.C.E.) broke this down further and created the atomic theory that the universe is made up of invisible, indestructible particles. The stream of thought started by the Pre-Socratics branched into several directions. Hippocrates (hih-PAW-kruh-teez) (ca. 470–400 B.C.E.) became the most prominent physician and teacher of medicine of his time. He sought natural explanations for diseases and natural means to treat them. 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.

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.

Socrates (SOK-ruh-teez) (ca. 470–399 B.C.E.), whose ideas are known only through the works of others, also applied philosophy to politics and to people. 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 rather than lecturing, a process known as the Socratic method. Many Athenians viewed Socrates with suspicion because he challenged the traditional beliefs and values of Athens. His views brought him into conflict with the government. The leaders of Athens tried him for corrupting the youth of the city, and for impiety, that is, for not believing in the gods honored in the city. In 399 B.C.E. they executed him.

Most of what we know about Socrates 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.

Plato’s student Aristotle (384–322 B.C.E.) believed that true 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. 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 earth and judged the earth to be the center of the universe, with the stars and planets revolving around it.

The philosophers of ancient Athens lived at roughly the same time as major thinkers in religious and philosophical movements in other parts of the world, including Mahavira (the founder of Jainism), the Buddha, Confucius, and several prophets in Hebrew Scripture. All of these individuals thought deeply about how to live a moral life, and all had tremendous influence on later intellectual, religious, and social developments. There is no evidence that they had any contact with one another, but the parallels among them are strong enough that some historians describe the period from about 800 B.C.E. to 200 B.C.E. as the “Axial Age,” by which they mean that this was a pivotal period of intellectual and spiritual transformation.