The philosophical grandfather of Classical Liberalism and the single most influential political thinker on the American founding. Against Thomas Hobbes's absolutist reading of the The Social Contract, Locke insisted that the state of nature, though inconvenient, is already governed by a natural moral law and that people possess natural rights to "life, liberty, and property" prior to any political authority. Governments are instituted to protect those rights; when they violate them systematically, people may resist — a doctrine Jefferson lifted almost verbatim for the Declaration of Independence.
Locke's other great contribution is epistemological. An Essay Concerning Human Understanding (1689) launched British empiricism: the mind at birth is a tabula rasa, all our ideas come from experience, and knowledge is the ordered comparison of those ideas. This picture of the knowing subject dovetailed neatly with his politics — individuals are the fundamental units of both knowledge and authority.
His blind spots are notorious: his theory of property was used to justify colonial expropriation, and he invested in the slave trade while writing elegantly about liberty. But the framework he built — consent, rights, limited government, religious toleration — became the operating system of the liberal tradition.