Because of his insistence on the need for an empirical component in knowledge and his antipathy to speculative metaphysics, Kant is sometimes presented as a positivist before his time, and his attack upon metaphysics was held by many in his own day to bring both religion and morality down with it. Such, however, was certainly far from Kant’s intention. Not only did he propose to put metaphysics “on the sure path of science,” he was prepared also to say that he “inevitably” believed in the existence of God and in a future life. It is also true that his original conception of his critical philosophy anticipated the preparation of a critique of moral philosophy. The Kritik der praktischen Vernunft (1788, spelled Critik and practischen; Critique of Practical Reason), the result of this intention, is the standard sourcebook for his ethical doctrines. The earlier Grundlegung zur Metaphysik der Sitten (1785; Groundwork of the Metaphysics of Morals) is a shorter and, despite its title, more readily comprehensible treatment of the same general topic. Both differ from Die Metaphysik der Sitten (1797; The Metaphysics of Morals) in that they deal with pure ethics and try to elucidate basic principles; the later work, in contrast, is concerned with applying these principles in the concrete, a process that involved the consideration of virtues and vices and the foundations of law and politics.

There are many points of similarity between Kant’s ethics and his epistemology, or theory of knowledge. He used the same scaffolding for both—a Doctrine of Elements, including an Analytic and a Dialectic, followed by a Methodology—but the second Critique is far shorter and much less complicated. Just as the distinction between sense and intelligence was fundamental for the former, so is that between the inclinations and moral reason for the latter. And just as the nature of the human cognitive situation was elucidated in the first Critique by reference to the hypothetical notion of an intuitive understanding, so is that of the human moral situation clarified by reference to the notion of a “holy will.” For a will of this kind there would be no distinction between reason and inclination; a being possessed of a holy will would always act as it ought. It would not, however, have the concepts of duty and moral obligation, which enter only when reason and desire find themselves opposed. In the case of human beings, the opposition is continuous, for humans are at the same time both flesh and spirit; it is here that the influence of Kant’s religious background is most prominent. Hence, the moral life is a continuing struggle in which morality appears to the potential delinquent in the form of a law that demands to be obeyed for its own sake—a law, however, the commands of which are not issued by some alien authority but represent the voice of reason, which the moral subject can recognize as his own.

In the Dialectic, Kant took up again the ideas of God, freedom, and immortality. Having dismissed them in the first Critique as objects that can never be known because they transcend human sense experience, he now argued that they are essential postulates for the moral life. Though not reachable in metaphysics, they are absolutely essential for moral philosophy.

Kant is often described as an ethical rationalist, and the description is not wholly inappropriate. He never espoused, however, the radical rationalism of some of his contemporaries nor of more recent philosophers for whom reason is held to have direct insight into a world of values or the power to intuit the rightness of this or that moral principle. Thus, practical, like theoretical, reason was for him formal rather than material—a framework of formative principles rather than a content of actual rules. This is why he put such stress on his first formulation of the categorical imperative: “Act only on that maxim through which you can at the same time will that it should become a universal law.” (Kant contrasted the categorical imperative, which holds absolutely or unconditionally, with hypothetical imperatives, which are valid only in the presence of some ulterior desire or goal—e.g., “If you want to be well-liked, do not lie.”) Lacking any insight into the moral realm, humans can only ask themselves whether what they are proposing to do has the formal character of law—the character, namely, of being the same for all persons similarly circumstanced.