In the opinion of Thomas Jefferson, three men—Francis Bacon, Isaac Newton, and John Locke—were the greatest of the great men who had ever lived. This, of course, reflected Jefferson’s commitment to the Enlightenment values that they did so much to further. The reputation of Locke (1632-1704) rested on two great books, his Second Treatise of Government and his Essay Concerning Human Understanding, which is excerpted here. Locke spent seventeen years pondering what questions the human understanding was capable of addressing. His great epistemological answer laid the foundation of the Anglo-American empirical tradition with its assertion that all babies are born with blank minds which their senses then fill up. In his Second Treatise, Locke argued that all men were endowed with natural rights that reason prompted them to secure with a mutual contract establishing government. In his Essay, he demonstrated philosophically the impact of the environment, a lesson that soon became linked to the idea of progress. Both masterpieces, these works shared the unusual fate of pushing political thought and philosophy in radical directions that were hardly imaginable before their time.