Plato and social contract theory

Plato asks what justice is by imagining a city where each part does its own work, while social contract thinkers ask what free people would agree to if no one ruled them yet. For Plato, justice is harmony of the soul and the city; for Hobbes, Locke and Rousseau, legitimacy comes from agreement. For example, Hobbes argues that even a harsh sovereign is better than a war of all against all.