John Locke (1632-1704) was a leading political philosopher during the Enlightenment, and a major contributor to liberal philosophy. His theories of the rights of man, including property, were adopted by the American founding fathers (the phrase "Life, liberty, and the pursuit of happiness" is almost directly taken from Locke [his original words were "life, liberty, and estate"] ). Locke described society as a contract between individuals called the "social contract", and held that the formation of collectives by individuals was the only way to ensure economic prosperity (see his Second Treatise on Government).
Locke's view helped lay the foundation for the constitutional republican form of government that we use in the United States. Locke had built on the prior work of fellow Englishmen Francis Bacon and Thomas Hobbes.
Locke is also famous for his theory of the tabula rasa, or "blank slate", which he is said to have first conceived while spending many hours of fruitless effort on a math problem as a boy.