Aristotle

Plato's student, and his opposite in method. Where Plato reasoned toward the ideal, Aristotle collected. He and his school gathered the constitutions of more than a hundred and fifty Greek states and asked what actually held them together and what tore them apart. The result is the first comparative political science. His definition of the citizen, one who rules and is ruled in turn, is still the cleanest sentence anyone has written about what membership in a republic asks of a person.