The concept of the right of revolution was also taken up by John Locke in Two Treatises of Government as part of his social contract theory. Locke declared that under natural law, all people have the right to life, liberty, and estate; under the social contract, the people could instigate a revolution against the government when it acted against the interests of citizens, to replace the government with one that served the interests of citizens. In some cases, Locke deemed revolution an obligation. The right of revolution thus essentially acted as a safeguard against tyranny.