But perhaps their most famous contribution is in terms of philosophy. And if you're going to talk about philosophy in ancient Greece, the most famous three philosophers are Socrates, Plato, and Aristotle. Now before we get into the first of them, and really the teacher of Plato, who was then the teacher of Aristotle, let's get a little bit of context on this time period. So as we enter into the fifth century BCE, a couple of things are emerging. First, you have a new Athenian democracy. Then as you get into that century, the Persians invade, try to conquer Greece several times, but unsuccessfully. But then the Greeks start fighting amongst themselves, led on one side by Athens and on the other side by Sparta, and that's the Peloponnesian War, which ends with Sparta being victorious, but all of the Greek city-states being weakened dramatically.