For contemporary Americans, one reason for studying Locke (together with Hobbes) is to understand the character of liberalism. A liberal system such as ours enshrines individual rights, but its health depends upon people exercising those rights responsibly. It depends on people taking seriously their duty to respect the rights of others.
Thomas Jefferson ranked Locke, along with Locke’s compatriot Algernon Sidney, as the most important thinkers on liberty. Locke helped inspire Thomas Paine’s radical ideas about revolution. Locke fired up George Mason. From Locke, James Madison drew his most fundamental principles of liberty and government. Locke’s writings were part of Benjamin Franklin’s self-education, and John Adams believed that both girls and boys should learn about Locke. The French philosopher Voltaire called “Locke the man of the greatest wisdom. What he has not seen clearly, I despair of ever seeing.”
Locke chose to avoid controversy by publishing his political writings anonymously. With the Two Treatises of Civil Government (1690) Locke established himself as a political theorist of the highest order. The First Treatise is a detailed refutation of the (now-forgotten) monarchist theories of Robert Filmer, but the Second Treatise of Government offers a systematic account of the foundations of political obligation. On Locke's view, all rights begin in the individual property interest created by an investment of labor.
Locke’s most famous work of political philosophy began as a reply to Filmer’s defense of the idea of the divine right of kings and ended up becoming a defense of natural rights, especially property rights, and of government limited to protecting those rights. This 1764 edition is famous for being the edition which was widely read in the American colonies on the eve of the Revolution.
The fundamental principles of Locke's philosophy are presented in An Essay Concerning Human Understanding (1690), the culmination of twenty years of reflection on the origins of human knowledge. According to Locke, what we know is always properly understood as the relation between ideas, and he devoted much of the Essay to an extended argument that all of our ideas—simple or complex—are ultimately derived from experience.
Locke wrote and developed the philosophy that there was no legitimate government under the divine right of kings theory. The Divine Right of Kings theory, as it was called, asserted that God chose some people to rule on earth in his will.
Locke imagined an original state of nature in which individuals rely upon their own strength, then described our escape from this primitive state by entering into a social contract under which the state provides protective services to its citizens. Unlike Hobbes, Locke regarded this contract as revokable. Any civil government depends on the consent of those who are governed, which may be withdrawn at any time.
Locke contended that prior to the political state there had existed a state of nature, in which human beings possessed rights to "life, liberty, and estate."
Locke believed that individuals should be free to seek their own truths in their own way, with their own minds, and with respect for the right of others to do so as well. His views on equality and toleration were based on respect for individuals' freedom to be guided by reason toward their own enlightenment.
John Locke is one of the founders of “liberal” political philosophy, the philosophy of individual rights and limited government. This is the philosophy on which the American Constitution and all Western political systems today are based. In the Second Treatise of Government, Locke’s most important political work, he uses natural law to ground his philosophy.