Late in his life, after his revolutionary work in epistemology, Kant first presented his mature moral philosophy in Foundations of the Metaphysics of Morals. Here, Kant developed his influential idea that human beings as rational agents are “autonomous,” or have the capacity for moral self-government. For Kant, autonomy means that, as rational beings, people set the own standards of conduct, as distinct from the demands made by their desires, and are able to decide and act on these standards. On the basis of a complex argument, Kant concluded that autonomy is possible only if the will is guided by a supreme principle of morality that he called the “categorical imperative.” Kant viewed this imperative as the product of reason and as the basis for determining moral duties. He expressed it in three basic formulations.

“Act only according to that maxim by which you can at the same time will that it should become a universal law.” Kant defined a maxim as a subjective principle on which a person intends to act, and a universal law as a principle that applies to everyone. Therefore, his formula of universal law demands that one act only on maxims that one can rationally will that everyone adopt. Kant provided the following example of how to use the formula: Suppose that a person must borrow money for a personal need and knows that he is unable to repay it. Is it morally permissible for him to act on the maxim of falsely promising to pay back a loan in order to get the loan? The formula tells that the person may act on the maxim if he can rationally will its universalization. The person cannot rationally will this because it would mean that people would no longer trust promises to repay loans, including his own. Kant added that the immorality of the maxim is clear in that the person really wants people to keep their promises so that he can be an exception to the rule for this one occasion.

“Act so that you treat humanity, whether in your own person or in that of another, always as an end and never as a means only.” For Kant, “humanity” refers to people’s uniquely human characteristics, their rational characteristics, including autonomy and the capacity to understand the world and to form and pursue life-plans. Thus, his formula of humanity demands that people always act so that they respect themselves and others as beings with a rational nature. In The Metaphysics of Morals, Kant used the formula of humanity to argue for a variety of duties to oneself and others. According to Kant, respect for rational nature in oneself implies that one ought not to destroy or deny one’s intellectual and moral capacities through suicide, drug abuse, lying, self-deception, or servility. It also implies that one must further one’s own rational nature through developing one’s natural talents and striving to become virtuous. Respect for rational nature in others involves that one ought not harm them and must uphold their individual liberty, but Kant discussed these duties as part of his legal and political philosophy. More exclusive ethical duties to others are that one must fulfill the duty of beneficence, contributing to the flourishing of rational nature in others, and that one must not deny people’s humanity through arrogance, defamation, or ridicule.

“All maxims . . . ought to harmonize with a possible realm of ends.” This formula shows that the two previous formulas are interconnected. (Kant held them all to be equivalent, but this has not been widely accepted.) Kant described the realm of ends as a harmony between human beings, resulting from each acting only on maxims that can become universal laws. It is a harmony of ends in that its members, by acting only on universalizable maxims, act only on maxims that can meet everyone’s consent; thus, they respect one another as rational self-determining agents, or ends in themselves. It is also a harmony of ends in that people will seek to further one another’s individual ends.

Kant held that people must mirror the realm of ends in their moral choices and actions, and that it is humanity’s duty to bring about this ideal. He viewed the French Revolution and the Enlightenment as steps in the right direction; argued for a worldwide league of democratic states as a further step toward the realm of ends; and claimed, moreover, that the religious institutions of his time must embrace the ideal, setting aside their historically evolved differences. Kant maintained that moral philosophy must not formulate new duties, but should only clarify the moral principle operative in “common moral reason” in order to help ordinary persons more adequately resist immoral desires. Kant’s clarification went beyond these confines and ended with an inspiring moral vision of the realm of ends as the purpose of history, the kingdom of God on Earth, and the ultimate individual and collective vocation.

Allison, Henry E. Idealism and Freedom: Essays on Kant’s Theoretical and Practical Philosophy. New York: Cambridge University Press, 1996. An important interpreter of Immanuel Kant explores relationships between Kant’s theory of knowledge and his moral philosophy.

Cassirer, Ernst. Kant’s Life and Thought. Translated by James Hayden. New Haven, Conn.: Yale University Press, 1981. Written by an important twentieth century philosopher, this book offers a readable intellectual biography of Kant.

Caygill, Howard. A Kant Dictionary. Cambridge, Mass.: Blackwell, 1995. A reliable reference guide that helps to clarify key concepts and ideas in Kant’s philosophy.

Copleston, Frederick. A History of Philosophy: Modern Philosophy. Garden City, N.Y.: Doubleday, 1964. Copleston devotes several lucid chapters to Kant and his significance in the history of philosophy.

Guyer, Paul, ed. The Cambridge Companion to Kant. Cambridge, England: Cambridge University Press, 1992. Helpful essays by contemporary Kant scholars shed important light on key aspects of Kant’s theory of knowledge, metaphysics, ethics, and religious thought.

Hare, John E. The Moral Gap: Kantian Ethics, Human Limits, and God’s Assistance. New York: Oxford University Press, 1996. A study of the strengths and weakness of Kant’s influential moral philosophy.

Hill, Thomas E., Jr. Dignity and Practical Reason in Kant’s Moral Theory. Ithaca, N.Y.: Cornell University Press, 1992.

Jones, W. T. A History of Western Philosophy: Kant to Wittgenstein and Sartre. New York: Harcourt, Brace, 1969. Provides a good starting point for readers who want a clear and basic introduction to Kant’s philosophy.

Kemp, John. The Philosophy of Kant. New York: Oxford University Press, 1968. A brief, readable account of Kant’s theory of knowledge, moral philosophy, and aesthetics.

Kuehn, Manfred. Kant: A Biography. New York: Cambridge University Press, 2001. The first major biography of the philosopher in fifty years. Includes extensive notes and a bibliography. For a review of this work see Magill’s Literary Annual review.

Schönfeld, Martin. The Philosophy of the Young Kant: The Precritical Project. New York: Oxford University Press, 2000. A study of the philosopher’s work before the Critique of Pure Reason.

Schott, Robin May, ed. Feminist Interpretations of Immanuel Kant. University Park: Pennsylvania State University Press, 1997. Essayists bring the perspectives of feminist scholarship to bear on Kant’s method and thought.

Sullivan, Roger J. Immanuel Kant’s Moral Theory. Cambridge, England: Cambridge University Press, 1989.

Timmons, Mark, ed. Kant’s Metaphysics of Morals: Interpretative Essays. New York: Oxford University Press, 2002. Includes seventeen essays by leading contemporary Kant scholars. The work covers such topics as Kant’s views on rights, punishment, practical reasoning, freedom, virtue, happiness, moral judgement, love, duties to oneself, and motivation.

Walker, Ralph. Kant. New York: Routledge, 1999. An excellent biographical introduction to the thoughts of the philosopher, clearly presented and requiring no special background. Bibliography.

Wolff, Robert Paul, ed. Kant: A Collection of Critical Essays. Garden City, N.Y.: Doubleday, 1967. Important scholars contribute essays on a wide range of themes and issues in Kant’s philosophy.

Yovel, Yirmiahu. Kant and the Philosophy of History. Princeton, N.J.: Princeton University Press, 1980.