Aristotle is the ancestor of political science as an empirical discipline and of virtue ethics as a moral one, and the two are connected. He held that the polis exists by nature because human beings are political animals, creatures who realize what they are only in a shared life under law. A person outside the city is either a beast or a god.
His method was to look first. He and his school collected the constitutions of 158 cities before generalizing, and the Politics reads as comparative government rather than as an ideal blueprint. That is the break with his teacher. Where the Republic asks what the best city would be, Aristotle asks which constitution suits which people, and answers that the most durable is polity, a mixture of oligarchy and democracy resting on a large middle class with enough property to be independent and not enough to despise their neighbors.
He also supplied the vocabulary that political argument still runs on: distributive justice as proportion rather than equality, the distinction between ruling for the common good and ruling for oneself, deliberation as the activity proper to citizens, and the claim that law should govern because law is reason without passion. Not all of it survives inspection. His defense of natural slavery is an argument he could have refuted with his own tools, and his account of women is worse.
The afterlife is enormous. The Arabic commentators made him the Philosopher; Averroes earned the title of Commentator; Aquinas Christianized the whole apparatus and made it the spine of scholastic politics. Every subsequent revival of virtue, the common good, or the political nature of human beings is a return to this text.