In 1973, nine years before he published his magnum opus in political philosophy, The Ethics of Liberty, Murray Rothbard issued a comprehensive popular presentation of the libertarian philosophy in For a New Liberty: The Libertarian Manifesto, first published by the mainstream publisher Macmillan.
The book is an excellent discussion of libertarian principles and applications, and it is still worth reading today. In rereading the book for the first time in decades, I found the foundational material especially interesting. Indeed, the material in For a New Liberty foreshadows what we find in greater detail in the later Ethics of Liberty.
As we saw in the later book, Rothbard believed that what he called the "nonaggression axiom" had to be derived. Although he used the word axiom, rather than principle or maxim or (as I prefer) obligation, he did not mean that the idea of nonaggression was self-evident, a priori, or self-justifying. Nor did he say that the denial of the axiom results in a contradiction.
As Rothbard wrote, "If the central axiom of the libertarian creed is nonaggression against anyone's person and property, how is this axiom arrived at?" Clearly, then, he regarded it as a derived principle. Does that mean he was wrong to call it an axiom? Not according to Roderick Long:
Another objection [to the nonaggression axiom] focuses on the term "axiom," which is sometimes taken to imply that the prohibition of aggression enjoys a special epistemic status analogous to that of the law of non-contradiction, e.g., that it is self-evident, or knowable a priori, or a presupposition of all knowledge, or that it cannot be denied without self-contradiction. While some proponents of the prohibition do indeed claim such a status for it, many do not, and accordingly it is sometimes suggested that "non-aggression principle" or "zero aggression principle" is a more accurate label than "non-aggression axiom."
On the other hand, there is a broader sense of "axiom" in which a foundational presupposition of a given system of thought counts as an axiom within that system of thought … even if it rests on some deeper justification outside that system; for example, Isaac Newton described his fundamental laws of motion as "axioms" within his deductive system of mechanics, yet regarded them as grounded empirically. In this sense non-aggression might legitimately be regarded as an "axiom" of libertarian rights theory regardless of what one takes its ultimate justification to be.
Rothbard continued his own discussion of the foundation of the nonaggression axiom thusly:
What is [the axiom's] groundwork or support? Here, libertarians, past and present, have differed considerably. Roughly, there are three broad types of foundation for the libertarian axiom, corresponding to three kinds of ethical philosophy: the emotivist, the utilitarian, and the natural rights viewpoint.
"Emotivists," he wrote, "assert that they take liberty or nonaggression as their premise purely on subjective, emotional grounds." He was undoubtedly dissatisfied with that "foundation":
While their own intense emotion might seem a valid basis for their own political philosophy, this can scarcely serve to convince anyone else. By ultimately taking themselves outside the realm of rational discourse, the emotivists thereby insure the lack of general success of their own cherished doctrine.
He meant that one must give reasons for why we all have a right not to be aggressed against, or why (changing perspective) we owe it to others to abstain from aggression.
He also dismissed utilitarianism as a foundation for libertarianism. While he agreed that freedom produces the good consequences claimed by utilitarians, he found this defense wanting because it is confined to consequences only and has led to a weak espousal of guidelines in political theory, rather than to "an absolute and consistent yardstick." He might have gone further and argued that strict consequentialism cannot contend with the fact that the various things that contribute to human well-being are discrete and incommensurable (there's no homogenous thing called well-being) and that interpersonal comparisons of subjective utility are impossible. In other words, the required utilitarian calculus cannot be executed.
That leaves the "natural-rights basis for the libertarian creed," which Rothbard claimed is the "basis which, in one form or another, has been adopted by most of the libertarians, past and present."
"Natural rights," he went on, constitute "the cornerstone of a political philosophy which, in turn, is embedded in a greater structure of 'natural law.'" From there, Rothbard provided material similar to what he would write later. He described human nature and the nature of the world as requiring that each person
learn about himself and the world, use his mind to select values, learn about cause and effect, and act purposively to maintain and advance his life. Since men can think, feel, evaluate, and act only as individuals, it becomes vitally necessary for each man's survival and prosperity that he be free to learn, choose, develop his faculties, and act upon his knowledge and values. This is the necessary path of human nature; to interfere with and cripple this process by using violence goes profoundly against what is necessary by man's nature for his life and prosperity. Violent interference with a man's learning and choices is therefore profoundly "antihuman"; it violates the natural law of man's needs.