How 5 Ancient Greek Philosophers Shaped Western Thought

Beginning in the sixth century B.C., a handful of intellectually curious inhabitants of ancient Greece began seeking non-mythological explanations for various phenomena in the world around them, as well as the origins of that world. Over the next two centuries, five of these trailblazing thinkers invented new ways to consider fundamental human problems and created many intellectual tools and frameworks we use to understand the world today.

Believed to be from the island of Samos, off the coast of modern Turkey, Pythagoras left no actual writings, and what little we know of him comes from the writings of later philosophers.

Many people may know Pythagoras’ name primarily from his namesake theorem, which determines the length of the hypotenuse of any right angle triangle. He and his followers (known as Pythagoreans) did not invent this theorem, but they may have been the earliest to provide mathematical proof, and to widely publicize it.

More importantly, Pythagoras is believed to have been the first to have thought about the natural world as a cosmos—a well-ordered, intelligible system—rather than a place ruled by chaos. He and his followers saw numbers and mathematics as the key building blocks of that system, a core belief that would lay the foundations for science.

“That’s a big deal,” says Eric Brown, associate professor of philosophy at Washington University in St. Louis. “The world is no longer a scary place where we have to tell stories about gods to make sense of what’s going on. There’s something that we can understand.”

The ideas of Heraclitus, who hailed from Ephesus, a coastal city not far from Samos, survive only in the short fragments that later authors quoted in their work. Like Pythagoras, Heraclitus believed in a cosmos, but in his case, the essential material that ordered that cosmos was fire. Later Greek philosophers known as the Stoics would develop this idea further, arguing that the world goes through a constant series of conflagrations.

Perhaps Heraclitus’ greatest contribution to early Greek philosophy was his view that the world is not a place of stable, unchanging truths—as was thought at the time he was writing. Instead, it’s in a state of constant flux or change, which he famously illustrated by comparing life to a river. Plato would later engage with the Heraclitean concept of flux in his famous dialogues and his responses to Heraclitus’ ideas have helped shape much of Western philosophical tradition.

“Plato agrees with Heraclitus that the perceptible world around us is subject to constant change,” Brown says. “Whereas he thinks the intelligible truths about the structure of the world—those things don’t change.”

What we know about Socrates comes from the works of three of his contemporaries in ancient Athens: Plato, his student and friend; the playwright Aristophanes and the historian Xenophon. During his lifetime, Socrates was a controversial figure who challenged the leading thinkers and political figures of his day with his constant questioning of their ideas. But Socrates was interested not in abstract cosmological speculation—he was concerned with practical questions about how to live a good life.

Brown recalls a famous line from the Roman statesman Cicero, in which he said that Socrates “brought philosophy down from the heavens.” “[In] contrast with people like Pythagoras and Heraclitus, Socrates was not so interested in trying to explain the cosmos,” Brown says. “He was interested in understanding how to live.”

Socrates’ pursuit of knowledge through questioning not only pioneered what would become known as the Socratic method—still used in many educational settings today—but eventually led to his trial and death sentence in 399 B.C. His ideas and teachings would live on in the writings of Plato and others who sought to emulate his model of intellectual inquiry.

“The most important thing about Socrates is this thought that there’s a distinct way of life, where you devote yourself to trying to understand how the world works, or how to live, or both,” Brown says. “That’s philosophy.”

Born into an aristocratic family in Athens, Plato grew disillusioned with Athenian politics after the trial and death of his most influential teacher, Socrates. Around 380 B.C., he founded his own Academy, considered the earliest ancestor of the modern university. Through his writings, nearly all of which take the form of dialogues, Plato used the Socratic method of inquiry to explore and develop his own comprehensive system of ideas about the natural world, reality, ethics and politics. “Plato is by far the biggest, most important Greek philosopher,” Brown says. “His dialogues more or less invent the idea of philosophy.”

Plato is particularly well known for his Theory of Forms, which argues that there is an underlying world of perfect, unchanging entities known as “Forms” (or ideas) beyond the highly flawed, constantly changing physical world we perceive with our senses. In one of his most well-known works, The Republic, he laid the foundations for political philosophy by setting out his theories about the ideal form of government and leadership.

Born in Stagira, a city in Macedonia, Aristotle studied with Plato in Athens for nearly 20 years. He later broke from some of Plato's ideas while retaining a focus on understanding the natural world through empirical observation and logical reasoning. “I sometimes tell my students that Aristotle is Plato, if you thought the key science was not math, but biology,” Brown says. “It’s something of an oversimplification, but it’s not totally distorting.”

Through his work, Aristotle founded the study of logic, formalizing the rules for how to think clearly and make an argument. With his extensive classification of animals according to genus and species, he laid the groundwork for modern-day zoology. Aristotle and his followers also conducted a study of various city governments, comparing the elements of each to develop a theory of the most just form of government.

Building on the work of Plato and the other philosophers who came before him, Aristotle developed his own way of understanding the real world through systematic and scientific study. “Aristotle’s approach to politics was his approach to biology,” Brown says. “You collect different specimens and you study them.”