The political theorist known for his work on the social contract is Jean-Jacques Rousseau. In his seminal book "The Social Contract," Rousseau discusses the idea that legitimate political authority arises from an agreement among individuals to form a collective society. He argues that people enter into this contract to safeguard their natural rights while agreeing to abide by the general will of the community. Rousseau’s ideas have profoundly influenced modern democratic thought, emphasizing the role of popular sovereignty and the responsibilities of both individuals and government in a democratic society.