From Practical Reason to the Seeds of Absolute Idealism

Kant's "Groundwork of the Metaphysics of Morals" (1785) formulates the Categorical Imperative: act only according to that maxim through which you can at the same time will that it become a universal law. Alongside this Formula of Universal Law, the Formula of Humanity demands treating rational beings always as ends in themselves and never merely as means, while the Formula of Autonomy grounds the moral law in the rational will's own self-legislation, making freedom, understood as autonomy, the ratio essendi (ground of being) of the moral law, and the moral law in turn the ratio cognoscendi (ground of knowing) of freedom. The "Critique of Practical Reason" (1788) then posits the Postulates of Pure Practical Reason — freedom, immortality, and the existence of God — as rationally required to make coherent the "highest good," the union of virtue with happiness. Kant's doctrine of the "fact of reason" (Faktum der Vernunft) holds that our consciousness of the moral law is not derived from any prior metaphysical proof but is simply given within ordinary moral consciousness, and his rigorism — the doctrine that duty admits no exceptions, epitomized by his notorious claim that one must not lie even to a murderer asking after a potential victim's whereabouts — became a lasting flashpoint of 19th-century ethical controversy.