Groundwork for the Metaphysic of Morals, published in 1785, is Kant’s first major work in ethics. Like the Prolegomena to Any Future Metaphysics, the Groundwork is the short and easy-to-read version of what Kant deals with at greater length and complexity in his Critique. The Critique of Practical Reason, published three years later, contains greater detail than the Groundwork and differs from it on some points—in the Critique of Practical Reason, for instance, Kant places greater emphasis on ends and not just on motives—but this summary and analysis will cover only the general points of Kant’s ethics, which both his major works share in common.

Morality applies to all rational beings, and a moral action is defined as one that is determined by reason, not by our sensual impulses. Because an action is moral on account of its being reasoned, the moral worth of an action is determined by its motive, or the reason behind the action, not by its consequences. We can determine the worth of the motive behind any given moral action by asking whether we could turn that motive into a universally applicable maxim. Reason is the same at all times and for all people, so morality too should be universal. Therefore, an action is moral only if it embodies a maxim that we could will to be a universal law.

Kant calls it a “categorical imperative” that we must act in such a way that we could will the maxim according to which we act to be a universal law. He contrasts this with the “hypothetical imperative,” which would demand that we act to achieve certain ends. The maxim of a hypothetical imperative would assert, “do such-and-such if you want to achieve such-and-such result.” There are no ifs in moral action, according to Kant. Morality works according to a categorical imperative because we must act in a given way simply because the motive is admirable, not because we have calculated that we can achieve certain ends as a result.

Once we recognize the universality of moral law, we must also recognize that it applies equally to all people. Acting morally, then, requires that we recognize other people as moral agents and always treat them as ends in themselves, not as means by which we can achieve our own ends. We must also ensure that our actions do not prevent other people from acting in accordance with moral law. Kant envisions an ideal society as a “kingdom of ends,” in which people are at once both the authors and the subjects of the laws they obey.

Morality is based in the concept of freedom, or autonomy. Someone with a free, or autonomous, will does not simply act but is able to reflect and decide whether to act in a given way. This act of deliberation distinguishes an autonomous will from a heteronomous will. In deliberating, we act according to a law we ourselves dictate, not according to the dictates of passion or impulse. We can claim to have an autonomous will even if we act always according to universal moral laws or maxims because we submit to these laws upon rational reflection.

Kant answers the tricky question of free will and determinism—how can we at once assert that we have a free will and that we live in a world that functions according to necessary physical laws?—by drawing on his distinction from the Critique of Pure Reason between the phenomenal world of appearances and the noumenal world of things-in-themselves. Physical laws apply only to appearances, whereas the will is a thing-in-itself about which we have no direct knowledge. Whether the will is actually free we can never know, but we still act in accordance with the idea of freedom.