One of the most famous English philosophers of the 17th century, John Locke, was also a physician and a political theorist. Locke believed that individuals are born with natural rights and is one of the founders of the liberal political philosophy. John Locke is best known for his book, An Essay Concerning Human Understanding, which contains his theory of ideas. His other popular works of writing include A Letter Concerning Toleration and Two Treatises of Government.