By the close of the 6th century BCE Athens was on the brink of civil strife among rival aristocratic families. Cleisthenes, with the backing of a popular movement, proposed a sweeping reorganisation that broke up the old kin-based political structure and replaced it with ten new "tribes" (*phylai*) drawn on geographic lines. The assembly adopted the reform in 508–507 BCE.

The result was a form of government the world had not seen before. The main decision-making body, the *Ekklesia* (assembly of citizens), was now open to all adult male citizens, each with the right to speak, to vote, and to propose laws. A 500-member *Boule* (council) was selected by lot; judicial cases passed to large popular juries. The institution of "ostracism" — writing a person's name on a potsherd to send them into ten-year exile — allowed the assembly to remove anyone deemed a threat to the polity.

The system was limited: women, foreign residents (*metoikoi*), and the large enslaved population were not citizens. Even so, the principle that "those who rule are those who are ruled" was put into operation at the scale of a state for the first time, becoming the reference point for almost all later Western political thought. Athenian democracy lasted some two centuries, ending gradually under Macedonian rule — but the idea endured.