Thomas Hobbes wrote Leviathan in 1651 in the middle of the English Civil War, with regicide three years past and the country still bleeding, and proposed that political authority is justified by an implicit contract between the governed and the sovereign — that without it, life would be solitary, poor, nasty, brutish, and short. John Locke in 1689 revised the contract to make government's authority conditional on its protection of natural rights, with revolution legitimate when the contract is broken; Jean-Jacques Rousseau in 1762 made it radically democratic by locating sovereignty in the general will. The American Founders read Locke and the French revolutionaries read Rousseau, and modern political philosophy has been a series of refinements on the same idea ever since.
Social-contract theory begins as a thought experiment: imagine a state of nature without political institutions, and ask what authority rational people inside it would consent to. Hobbes's state of nature is intolerable — a war of all against all — so rational individuals would consent to almost any sovereign who could provide order, and the contract creates the Leviathan. Locke made the contract conditional: people have natural rights (life, liberty, property) but lack an impartial judge, so they consent to government to secure those rights, and if government violates them, revolution is legitimate — the American Declaration of Independence is essentially Locke filtered through Jefferson. Rousseau turned the framework inside out: the primitive human is naturally good but corrupted by society, and the social contract should return something approximating that goodness through democratic institutions in which the general will — distinct from the mere will of all — is sovereign. The standard objections cluster around four points: no actual contract exists (defenders reply that it is normative); initial bargaining inequalities can ratify themselves into the agreement (Marxist critique); the framework is conspicuously Western; and it ignores groups and shared identities (communitarian critique). The modern reformulation runs through John Rawls and his veil of ignorance (1971), and through Nozick's Lockean defense of the minimal state and Gauthier's Morals by Agreement. Across all variants the question is the same: what could legitimate political coercion to someone who could refuse it?
The social contract is the framework most modern democracies invoke explicitly — the U.S. Declaration of Independence (Lockean rights and popular consent), the French Declaration of the Rights of Man (Rousseau's general will), the UN Universal Declaration of Human Rights of 1948 — and constitutional law still operates within its assumptions of consent, bounded authority, and protection against state overreach. The framework is now being stressed by problems Hobbes did not imagine: climate change (future generations are not represented), immigration (does the contract bind states toward outsiders?), the platform economy, and AI (will systems eventually be parties or subjects?). Four centuries on, Hobbes's scaffold is still the dominant framework for thinking about legitimate political authority.