According to A.P. d’Entrèves (an important historian of political thought), “Kant was indeed the most forceful exponent of natural law theory in modern days,” and as such he was also “the most coherent and persuasive critic” of legal positivism, according to which the moral authority of law derives entirely from the will of the sovereign. (Natural Law: An Introduction to Legal Philosophy, 2^nd ed., 1970, p. 110.) Similarly, the Kantian scholar John Ladd noted that Kant’s theory of justice “is identical with what is generally known as natural law.” (Introduction to MEJ, p. xvii. Works written by Kant are signified by initials. See the conclusion of this essay for bibliographic details.)  

Natural law theories of ethics and justice go back to the ancient Greeks, and there are variations within this tradition. Generally speaking, however, natural law theories maintain that ethical and political principles can be justified by reason alone, that they are objective and universal in scope, and that they do not depend on the subjective feelings or desires of individuals or originate in the decrees of government. Modern libertarian thought, as found in the writings of John Locke and other classical liberals, emerged from this natural law perspective, and it remains a dominant theme in contemporary libertarian thought.

Immanuel Kant vigorously upheld the objective validity of fundamental moral and political principles; and, as I briefly explained in my last essay, he intended his Categorical Imperative to be a formal test that tells us which moral principles qualify as objectively justifiable and which do not. The Categorical Imperative is essentially a principle of universalizability, according to which moral principles must apply in the same way to all rational beings, without exception. We may not demand that others do x while exempting ourselves from the same rule, nor may we exempt others from the same moral standards that we apply to ourselves. As Kant put it:  “The first principle of morality is, therefore, act according to a maxim which can, at the same time, be valid as universal law.—Any maxim which does not so qualify is contrary to morality.” (DV, p 25.)

Among his three famous formulations of the Categorical Imperative, it is the second that has the most relevance to Kant’s theory of rights and justice. This second formulation reads as follows: “Act in such a way that you always treat humanity, whether in your own person or in the person of any other, never simply as a means, but always at the same time as an end.” (GMM, p. 91.) As an example of this principle, Kant discussed a person who violates the freedom or property rights of other people. “[I]t is manifest that a violator of the rights of man intends to use the person of others merely as means without taking into consideration that, as rational beings, they ought always at the same time to be rated as ends….” (GMM, 92.) That every human being (indeed, every rational being, whether human or not) is an end in himself, with the right to pursue happiness in his own way without the coercive interference of others, was an essential element of Kant’s moral and political theory. Quoting John Ladd once again:

The key to Kant’s moral and political philosophy is his conception of the dignity of the individual. This dignity gives to man an intrinsic worth, a value sui generis that is “above all price and admits of no equivalent.”…Kant may be regarded as the philosophical defender par excellence of the rights of man, of his equality, and of a republican form of government….A theme throughout his political writings is the condemnation of the use of violence and fraud [in social relationships]…(Introduction to MEJ, p. ix).

Kant’s theory of justice is most fully developed in The Metaphysical Elements of Justice, which comprises Part One of The Metaphysic of Morals (not to be confused with his earlier work Groundwork [or Foundation] of the Metaphysic of Morals.) It is highly significant that the second part of The Metaphysic of Morals is titled The Doctrine of Virtue, for Kant’s division between justice and virtue signifies his desire to distinguish between those spheres of human action in which coercion may legitimately be used and those spheres in which persuasion alone is morally justifiable. And it is this distinction that determines the moral limits of governmental power. Like classical liberals before him, Kant understood that governments primarily use coercion, not persuasion, to achieve their ends, so we must first determine when coercion is morally justifiable and when it is not if we are to understand the proper limits of government.

Kant drew a Bright Line between duties of virtue and duties of justice. The latter—juridical duties, such as respecting the rights of others and fulfilling our voluntary contracts—may legitimately be enforced by physical coercion, whereas the former cannot. According to Kant, “What essentially distinguishes a duty of virtue from a juridical duty is the fact that external compulsion to a juridical duty is morally possible, whereas a duty of virtue is based on free self-constraint.” (DV, p. 41.)

Before proceeding, I should clear up a potential misunderstanding. In distinguishing between justice and virtue, Kant did not mean to deny that justice itself must be based on objective moral principles. By “virtue,” in this context, he meant personal moral qualities and actions that do not violate the rights of others. Kant was drawing essentially the same distinction that we find in the title of Lysander Spooner’s great essay Vices are not Crimes. Matters of justice pertain to external actions that violate the rights of others, whereas virtuous actions emerge from the inner qualities (which Kant characterized as a “good will”) of individuals. Virtuous actions cannot, and therefore should not, be compelled by others; they must be freely chosen by each person, who is an autonomous moral agent. By this Kant meant that each person must “legislate” his own moral principles and act upon those principles voluntarily. In Kant’s approach, to speak of a virtuous action that one has been compelled to perform is a contradiction in terms.

Justice differs from virtue (in the sense I have just explained) because we may rightfully compel others to respect individual rights. Of course, some people may respect rights from a virtuous motive, as a matter of moral principle, in which case coercion is unnecessary. But humans are morally imperfect, so some people would rather use force instead of persuasion when dealing with others. In such cases coercion (physical force or the threat of force) is morally appropriate, because to violate a right is to violate the equal freedom of others, as mandated by the Categorical Imperative. Rights are a matter of law, as enforced by a government, and from this perspective it is irrelevant whether a person respects rights because he believes this to be morally required or because he fears the coercive consequences of violating rights.

Kant’s interest in establishing a Bright Line between the voluntary sphere of social interaction and the coercive sphere of governmental actions was by no means original with him. On the contrary, determining the exact nature of this Bright Line had been a major concern of liberals and libertarians for many years, as we see in John Milton’s statement about the basic goal of political philosophy: “[H]ere 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.” Or as John Locke expressed the same idea several decades later: “[I]t is one thing to persuade, another to command; one thing to press with arguments, another with penalties.” Likewise, 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.”

Kant’s attempt to justify a Bright Line between the proper spheres of persuasion and coercion placed him squarely in the tradition of liberal individualism, but he explored this problem in greater detail than had his predecessors. Indeed, Kant wrote an entire book on this subject—the aforementioned work, The Metaphysic of Morals—in which the differences between coercive justice and voluntary virtue are explained in considerable detail. Although Kant reached essentially the same conclusions in this area as had previous classical liberals, his method of argument differed from theirs substantially in several respects. I will explain some details of Kant’s political theory and his method of establishing the Bright Line in my next essay. For the remainder of this essay I will discuss some other points about the Categorical Imperative, especially its second formulation (quoted above), which states that we should always treat people as ends in themselves, and not merely as means to our ends.

It should be noted that in his second formulation of the Categorical Imperative, Kant maintained that we should treat not only others as ends in themselves but ourselves as well. This led Kant to argue that we have moral obligations to ourselves, including the obligation to preserve our lives and the obligation to strive for moral perfection to the best of our ability. (Moral perfection, in this context, pertains to personal qualities, or virtues, like integrity and honesty.) I cannot pursue this feature of Kant’s thinking in detail here. I called attention to Kant’s belief in duties to oneself, which if followed will enhance one’s self-esteem, to rebut the common belief among Objectivists that Kant was an advocate of pure selflessness. True, as I noted in my last essay, Kant did not regard actions motivated by self-interest as moral in nature, but he did not single out self-interest in this regard. Any action taken to satisfy a desire, whether that desire be to benefit ourselves or others, does not qualify as moral in nature, according to Kant. But this does not mean that hypothetical imperatives (as Kant called them), which take the form of “If you want x, then you should do y,” are immoral. On the contrary, such practical maxims, whether selfish or altruistic, may be perfectly consistent with moral principles, but they are motivated by prudential rather than by moral considerations. Only if we take an action because we believe that action is morally right, as a matter of principle, will our motive qualify as moral. It is therefore essential to understand that “moral,” in this context, should be contrasted with “nonmoral, not with “immoral.”

Although I personally regard the distinction between the moral and the prudential, which has become standard fare in the writings of many modern moral philosophers, as misconceived at its root—for one thing, it has frequently been used to argue that ethical egoism does not even qualify as a legitimate moral theory—we should not suppose that Kant used it to condemn self-interested actions. That was not his point at all. Rather, his Categorical Imperative, according to which all moral principles must be universalizable, is an unconditional demand of practical reason; that is to say, the Categorical Imperative applies (or should apply) regardless of what our personal desires may be. In contrast, hypothetical imperatives (You should do x if you want y) are conditional. Prudential maxims will vary according to circumstances and the goals and desires of particular individuals, and such maxims, which are highly contextual, may admit exceptions. Thus, since hypothetical imperatives, such as the prudential maxims that will promote personal happiness, cannot be applied universally to every human being, they do not meet Kant’s formal test for moral laws, as expressed in his Categorical Imperative. (For an indication of my own approach to the role of moral principles in human action, see my essay Jack and Jill and Two Kinds of Freedom.)

At this point, some of my readers may feel a headache coming on, as they wonder why they should care about Kant’s Categorical Imperative, his distinction between hypothetical and categorical imperatives, and other technical matters. Well, those who have read my recent essays will understand that my discussion of Kant has been largely defensive, insofar as I have attempted to correct some of the misunderstandings and egregious misrepresentations of Kant found in the writings of Ayn Rand, Leonard Peikoff, and other Objectivist philosophers. Now, if you don’t care about Ayn Rand or what she thought about Kant, then it is possible that my discussion of Kant’s moral philosophy won’t interest you very much, if at all. But as indicated by my earlier remarks about Kant’s theory of rights, he had some very interesting things to say about the moral foundations of a free society; and his arguments, even if we disagree with them, merit careful consideration. Moreover, as we shall see in my next essay, Kant sometimes took what I regard as the wrong position in controversies that had haunted classical liberals for many decades. A good example of this is Kant’s position on the rights of resistance and revolution against unjust and tyrannical governments. Kant denied these rights, and his opposition was even more absolute than the opposition expressed by David Hume, Adam Smith, and other classical liberals with a conservative bent. But Kant almost always defended his political positions with interesting and intriguing arguments, so he is worth reading for that reason alone.  

The following are the sources for the quotations from Kant used in this essay.

DV: The Doctrine of Virtue: Part II of the Metaphysic of Morals, trans. Mary J. Gregor (Harper, 1964).

GMM: Groundwork of the Metaphysic of Morals, translated and analyzed by H.J. Paton, in The Moral Law (Hutchinson, 1972).

MEJ: The Metaphysical Elements of Justice: Part I of the Metaphysics of Morals, trans. John Ladd (Bobbs-Merrill, 1965).