The third great The Social Contract theorist and the most dissonant voice in the Enlightenment chorus — a philosopher who helped build modernity and loudly mourned what it was costing. Rousseau's opening move in the Discourse on Inequality (1755) is to agree with Thomas Hobbes and John Locke that we can usefully imagine a state of nature, then to insist both misread it: pre-social humans were neither warlike nor proto-bourgeois but simply free, healthy, and indifferent. What corrupted us was society itself — property, comparison, amour-propre (the vanity born of other people's eyes).
His constructive answer, The Social Contract (1762), tries to square the circle: how can we live with others and remain free? Only, he argues, by each giving ourselves wholly to a political community and obeying only the "general will" — the law we give ourselves as citizens. This is the root of modern republican and democratic thought, but also the formulation that later critics (notably Isaiah Berlin) have read as an unsettling licence for forcing people to be free.
Rousseau was also a novelist, composer, educational theorist, and confessional memoirist. His influence ran through Immanuel Kant, German Romanticism, the The French Revolution, and nearly every critique of bourgeois society down to Karl Marx.