John Locke was an English philosopher and physician who is widely regarded as one of the most influential Enlightenment thinkers. He is best known for his work on the social contract theory, which he wrote about in his book 'Two Treatises of Government'. He argued that people have natural rights to life, liberty, and property, and that governments should be formed to protect these rights. He also argued that governments should be based on the consent of the governed, and that citizens should have the right to revolt if their government fails to protect their rights.