As is my wont, I will spend much of this essay (and some of the next) sketching some historical background on the theory of natural rights. I do this not because I am a history buff (though I am certainly that) but because it can be nearly impossible to appreciate a philosophical theory without some knowledge of its provenance. In short, I believe that the theoretical justification for natural rights is interwoven throughout the history of that doctrine. But history is not everything, so in my next essay I shall consider some purely theoretical issues.

There has recently been considerable discussion on Lib​er​tar​i​an​ism​.org about the non‐​aggression principle (NAP) or, alternatively, what Randians frequently call the non‐​initiation of force (NIOF) principle. I trust my readers already understand that neither qualifies as a moral foundation of libertarianism. They are merely ways of formulating a premise that all (or most) libertarians share. If you subscribe to the NAP or to the NIOF principle, then you probably qualify as a libertarian. But there remain the questions: How can these principles be justified? What is their moral foundation?

A theory of natural rights, in my judgment, is the moral foundation of libertarianism, so let’s begin with a clarification of terms.

Historically, the “natural” in “natural rights” referred to two different but interrelated things: first, to moral principles that are based on the nature of man and social interaction, in contrast to rights that are created by customary or man‐​made (“positive”) law; second, to moral principles that can be discovered and justified by the natural faculty of reason alone, without the assistance of divine revelation.

In modern libertarian thought (and in political philosophy generally), to speak of “a right” is to speak of an enforceable moral claim. Thus when we say that a right has been “violated,” we mean that the victim has the option of using physical force against the violator, whether in self‐​defense or for the purpose of restitution and/​or punishment.

The most fundamental problem in the history of classical liberal and libertarian thought was to draw a bright line between persuasion and coercion. (There were various reasons for this concern, but the first and most urgent problem had to do with the desire for religious freedom.) As John Milton put it in the seventeenth century: “here the great art lies, to discern in what the law is to bid restraint and punishment, and in what things persuasion only is to work.” A few decades later John Locke insisted that “it is one thing to persuade, another to command; one thing to press with arguments, another with penalties.” And in the following century, Adam Smith wrote: “We must always carefully distinguish what is only blamable, or the proper object of disapprobation, from what force may be employed either to punish or to prevent.”

The theory of natural rights was specifically designed to establish a bright line between persuasion and coercion. Rights define and delimit the legitimate use of physical force (or the threat of force) in social interaction. Rights are therefore essential elements in the libertarian theory of justice. To violate a right is to commit an injustice. Justice is a “negative” virtue insofar as it demands only that we abstain from using coercion when dealing with other people. As Adam Smith put it in The Theory of Moral Sentiments, “We may often fulfill all the rules of justice by sitting still and doing nothing.”

Libertarianism does not say how we should live our personal lives; it merely says that if we choose to interact with others, we should do so by voluntary means. And it goes on to apply this basic social imperative to the political sphere by following a procedure that I call political reductionism. This means that all legitimate political rights must ultimately be reducible to the rights of individuals. Individuals do not gain special or exclusive rights by combining into groups or by calling themselves a “government.” Thus if a government coerces people in the name of a “right” that no individual could possibly possess, then we know that it is acting unjustly.

Libertarianism, as I said, is concerned with matters of justice, not with personal ethics. Merely because an act is just does not make it moral; this is the essential distinction between “crimes” and “vices.” A sleazy, lowlife character may treat others abysmally, but so long as he does not violate their rights, then his reprehensible conduct should be dealt with by means of voluntary social sanctions, not by means of coercive laws.

Like many libertarians of my generation, I came to libertarianism through the writings of Ayn Rand; and despite some disagreements I still regard her essay “Man’s Rights” as one of the finest brief treatments of this topic ever written. But there are a number of significant similarities between Rand’s approach and those of previous natural law philosophers, such as Hugo Grotius and Samuel Pufendorf. I am not suggesting that Rand borrowed from those sources; on the contrary, she was obviously unfamiliar with them. But as John Hospers once observed, Rand reinvented a number of wheels in moral and political philosophy, and she showed remarkable ingenuity in doing so.

In the early modern period, natural law and natural rights were often discussed in the context of the “law of nations,” or what Jeremy Bentham later called “international law.” The ultimate purpose of such works was to establish objective criteria for a just war. It is therefore understandable why detailed discussions of such criteria became especially popular and influential with the major work by Hugo Grotius in 1625, for this was published during the Thirty Years’ War (1618–1648), one of the most destructive conflicts in European history.

Those modern philosophers who scoff at the notion of a “state of nature,” and who lecture us about its unhistorical nature, often fail to appreciate why the state of nature was such an urgent topic of discussion among seventeenth‐​century philosophers. John Locke expressed the prevailing view of his contemporaries when he wrote: