Immanuel Kant is generally considered to be one of the most important philosophers of all time. His philosophy is often described as the culmination of the Enlightenment, providing a synthesis of rationalism and empiricism. His work is highly systematic, spanning most areas of philosophy. Kant’s theories had and continue to have a huge impact on many areas of philosophy. He is generally seen as a classical liberal, and he inspired and influenced many libertarian scholars, such as F. A. Hayek. Kant was much concerned with freedom and the prevention of coercion. His categorical imperative gives rise to a deontological ethical system. He argued that there are absolute constraints on what people are morally allowed to do. Certain kinds of actions are prohibited, and individuals have correlative negative rights that entitle them not to be interfered with in certain ways. These rights demarcate a sphere of noninterference and, hence, may be classified as absolute side constraints in the sense described by Robert Nozick. Although Kant’s ethical system is congenial to libertarianism, his political philosophy, while assigning a significant role to freedom, is at times at odds with classical liberalism. However, arguably these aspects of Kant’s political theory do not follow from his moral philosophy.

Kant believes that there is an objective standard of right and wrong that holds universally. His account of right and wrong, however, differs radically from most other ethical theories because it is not based on the commands of God, on some supposed teleological purpose of nature, on self‐​interest, on moral sentiments, or on a substantive conception of human nature. Rather, it is based on reason. For Kant, the moral law is a law of reason.

In the Groundwork of the Metaphysics of Morals, Kant sets out to identify the supreme principle of morality. That principle is the categorical imperative, which constitutes the centerpiece of Kant’s ethical theory. Kant’s argument begins with an analysis of the concept of the good will. According to Kant, the only thing that has unconditional worth is a good will. The worth of everything else is in some way dependent on the worth of a good will. Now a good will is understood as a will that acts out of duty. Acting out of duty has to be distinguished from acting in accordance with duty. The latter amounts to an action conforming to what duty commands, which, for Kant, is not sufficient for moral worth. Rather, for an action to be moral, the agent has to do the right action because it is right. That is, the rightness or morality of the action must feature in the motivation of the agent. It is not sufficient that he acts in a way that happens to be the right way. This argument would be too contingent and would make morality a matter of chance. Instead, there must be an internal connection insofar as the rightness of the action is the reason for the action. Kant then identifies acting out of duty with acting out of respect for universal law.

The most famous formulation of the categorical imperative is what is usually known as the “formula of universal law,” which states: “I ought never to act except in such a way that I could also will that my maxim should become a universal law.” The categorical imperative binds, as its name suggests, categorically and unconditionally. That is, its commands are binding on us in and of themselves. It tells us what to do, and its validity is not dependent on any desires or ends that we might have. There is no contingency involved. As such, it sharply differs from hypothetical imperatives that are only hypothetically binding, that is, binding on the condition that certain ends are willed. The commands of the categorical imperative also have categoricity, and the constraints that they impose are absolute. It does not admit of what has been called a utilitarianism of rights because the rights that it confers cannot be outweighed by the rights of other persons or by considerations of utility. Accordingly, it leads to a deontological moral system involving an understanding of rights as absolute side constraints on actions. The key idea that is embodied in the categorical imperative is the idea of universalization. It is a formal principle that tests maxims to assess whether they are universalizable, ruling out those maxims as inadmissible that fail this procedure.

Another formulation of the categorical imperative is the formula of humanity, which states: “so act that you use humanity, whether in your own person or in the person of any other, always at the same time as an end, never merely as a means.” This variant emphasizes not so much the formal nature of moral maxims, but rather the value of rational beings. There is a natural equality of human beings, deriving from the fact that they all possess the faculty of reason and are all capable of moral action. There is no natural hierarchy. Instead, every rational being possesses dignity. This dignity places important constraints on how we can treat other people. We have to respect their dignity and rationality, which implies that we cannot use them as mere means toward the achievement of our ends. Rather, we have to respect their rationality and treat them as ends in themselves. That is, we have to respect that they are autonomous agents who have their own ends to pursue.

These formulae of universal law and humanity, taken together, give us the “formula of the kingdom of ends.” “Act in accordance with the maxims of a member giving universal laws for a merely possible kingdom of ends.” This formula represents the combination of both the form and matter of morality. Although the idea of a universal law captures the form of morality, namely that it holds universally and with necessity, the idea of humanity provides the matter or end of morality insofar as it is rational beings who possess value, can act morally, and impose constraints on the actions of others. The idea of a kingdom or realm of ends integrates these two elements to give us a complete specification of the moral law. Moreover, it provides an inspiring ideal that can be understood as the state of affairs that would arise if everyone were to follow the commands of morality and toward the achievement of which we should continuously strive.

In the Metaphysics of Morals, Kant identifies two aspects of practical philosophy, namely the “doctrine of right” and the “doctrine of virtue.” The former is concerned with enforceable duties that are owed to other people and that bring with them correlative rights. Everyone to whom these duties are owed has a claim on us. Enforceability is restricted to negative duties. We have a narrow core of enforceable morality, combined with a wide base of ethical duties. Coercion can only be legitimately used when it comes to enforcing negative duties and upholding their correlative negative rights. Actions are assessed regarding their conformity to the laws, and external conformity is all that is demanded. Someone who follows the law purely out of self‐​interest without any regard to morality is behaving in a manner completely consistent with right. This notion is contrasted with virtue, which does not just require external conformity, but also takes into account the reasons for which we perform actions. Motivations matter for virtue, and an action is only virtuous if it was done because it was the right action.

Kant argues that the state exists to protect the freedom of the individuals, as well as to uphold right and property. There are no substantive ends that the state should achieve. Human interests and needs, and consequently socioeconomic factors, are not to be taken into consideration. Rather, the understanding of state activity is purely procedural and formal. It is lawfulness and universality that determine the concept of right. The state should only be concerned with the external sphere of action, with how people behave toward each other, ensuring that they do not interfere with each other’s external freedom. Individuals should enjoy complete inner freedom. Anything that does not impact on the freedom of other people, such as a person’s interests, needs, beliefs, and convictions, is to be left untouched by the state. Government has no business whatsoever in telling people what to do with their lives except insofar as it affects the freedom of others. Kant is strongly opposed to any form of paternalism. As we have noted, the distinction between acting in accordance with duty and acting out of duty is central to Kant’s ethics. Only the latter way of acting has moral worth. Hence, morality is only possible through freedom; it cannot be imposed from the outside, but must be freely chosen. Virtuous behavior cannot and should not be coerced or enforced. Liberty, considered as the absence of coercion, is thus a precondition of autonomy and moral agency.

Kant provides a defense of private property rights while also critiquing Locke’s theory that property arises from the mixing of one’s labor with the natural world. According to Kant, anything that does not have a will can be treated as a mere means and can be utilized for achieving the ends of rational beings. Any limitations on human actions must derive from the formal law of reason. Thus, the private acquisition of external goods is justified or authorized to the extent to which it is universalizable. This universalizability requires that acquisition be such as to make general relations of property possible, which for Kant implies the need for an institutionalization of property. Authorized appropriation thus implies that the property rights must be confirmed by a judicial or legislative procedure in which the universal agreement of all those who are affected is given. Prior to such confirmation, property rights are only subjectively justified, but they are not yet objectively determined. Thus, there is a need for a state to render property rights determinate. Property rights must be given a fixed and determinate standard by positive law that is legislated by the state and that represents the agreement of the general will.

Although this theory of right is in general in broad agreement with some of the key tenets of classical liberalism, there are certain illiberal strands in Kant’s thought. In particular, there is the noteworthy fact that Kant argues that there is no right to revolution. Individuals have rights against the rulers, but these are unenforceable. The prohibition on revolution is absolute, and reform is supposed to be peaceful and gradual. Active resistance against a political regime is not permitted under any circumstances. In this respect, Kant strongly diverges from Lockean and even Hobbesian political theory. Another feature that appears to contravene liberal doctrine is the restriction of full political membership to self‐​sufficient persons, that is, to property owners. This idea clashes with the universalism that is advocated by classical liberals and that features so strongly in Kant’s ethical thought. This restriction is symptomatic of a more general clash between the radical conception of morality and human nature that Kant advocates in his ethical writings and his rather submissive attitude toward politics, which results from the fact that what happens in practice usually does not match up to Kant’s highly rationalistic understanding of the state. Arguably, these illiberal tendencies do not follow from his philosophical system, but rather can be seen as personal prejudices and ad hoc additions that even conflict with his ethical commitments. That is, these problematic features partly derive from Kant’s failure to consistently turn his ethical theory into a political theory, rather than being integral to his thought.

In his well‐​known pamphlet, Toward Perpetual Peace, Kant argues that the highest political good is perpetual peace. It is a utopian aim that we can and should try to approximate. States have a duty to leave the international state of nature and enter a world republic in the same way that individuals who find themselves in a state of nature have a duty to enter into a political state. Such a world republic is an ideal that is practically unattainable. Kant argues that it can nonetheless be approximated and that a pragmatic substitute for it consists in a free federation of states that submit to international laws to ensure a condition of permanent peace. Peace among nations requires, according to Kant, that certain internal conditions obtain within the different states. That is, lasting external peace between states presupposes certain political arrangements of states. In particular, Kant thinks that states should have a republican form of constitution because he believes that states are more likely to be peaceful if they have representative governments.

Further Readings

Allison, Henry E. Kant’s Theory of Freedom. Cambridge and New York: Cambridge University Press, 1990.

Kant, Immanuel. Practical Philosophy. Cambridge and New York: Cambridge University Press, 1996.

Korsgaard, Christine. Creating the Kingdom of Ends. Cambridge and New York: Cambridge University Press, 1996.

Kuehn, Manfred. Kant: A Biography. Cambridge and New York: Cambridge University Press, 2001.

O’Neill, Onora. Constructions of Reason: Explorations of Kant’s Practical Philosophy. Cambridge and New York: Cambridge University Press, 1990.

Williams, Howard, ed. Essays on Kant’s Political Philosophy. Chicago: University of Chicago Press, 1992.

———. Kant’s Political Philosophy. New York: St. Martin’s Press, 1983.

Wood, Allen W. Kant’s Ethical Thought. Cambridge and New York: Cambridge University Press, 1999.

Originally published