Rothbard, The Ethics of Liberty: Natural Law and Natural Rights, addresses the pursuit of happiness as a natural right.
If, as we have seen, natural law is essentially a revolutionary theory, then so a fortiori is its individualist, natural-rights branch. As the nineteenth-century American natural-rights theorist Elisha P. Hurlbut put it:
The laws shall be merely declaratory of natural rights and natural wrongs, and . . . whatever is indifferent to the laws of nature shall be left unnoticed by human legislation . . . and legal tyranny arises whenever there is a departure from this simple principle.
A notable example of the revolutionary use of natural rights is, of course, the American Revolution, which was grounded in a radically revolutionary development of Lockean theory during the eighteenth century. The famous words of the Declaration of Independence, as Jefferson himself made clear, were enunciating nothing new, but were simply a brilliantly written distillation of the views held by the Americans of the day:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness [the more common triad at the time was “Life, Liberty and Property”]. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed. That whenever any form of Government becomes destructive of these ends, it is the Right of the people to alter or to abolish it.