Natural rights theory holds that individuals have certain rights–such as the rights to life, liberty, and property–in virtue of their human nature rather than on account of prevailing laws or conventions. The idea of natural rights reaches far back in the history of philosophy and legal thought. Arguably, it was already recognized in nascent form by ancient Greek thinkers such as Aristotle in the fourth century B.C., who argued that citizens who are equal by nature have the same natural right (that is, just claim) to political office (Politics III.16.1287a8–14). During the Middle Ages the concept of natural rights began to emerge in a more recognizably modern form. Medieval canon lawyers, philosophers, and theologians entered into heated debate over the status of individual property rights, with some contending that the right to property was natural and others that it was merely conventional.

In the early modern era, theories of natural rights were advanced by seminal thinkers, including Hugo Grotius, Thomas Hobbes, and Samuel Pufendorf. The most influential of these was the English philosopher John Locke, especially in his Second Treatise of Government published in the late seventeenth century. 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.” “The State of Nature has a Law of Nature to govern it,” he wrote, “which obliges every one: And Reason, which is that Law, teaches all Mankind, who will but consult it, that being all equal and independent, no one ought to harm another in his Life, Health, Liberty, or Possessions.”