Jean-Jacques Rousseau was the philosopher known for his social contract theory, which posited that individuals consent to form societies and governments in exchange for mutual protection and social order. His ideas significantly influenced modern political thought, emphasizing concepts of democracy, individual freedom, and the importance of the collective will. Rousseau's work laid foundations for later political movements and ideologies, including socialism and nationalism. His belief that government legitimacy comes from the consent of the governed remains a cornerstone of democratic theory.