From Deduction to Deed: Kant's Grounding of the Moral Law

In the Critique of Practical Reason, Kant presents the moral law as the sole ‘fact of pure reason’ that neither needs nor admits of a deduction to establish its authority. This claim may come as a surprise to many readers of his earlier Groundwork of the Metaphysics of Morals. In the last section of the Groundwork, Kant seemed to offer a sketch of just such a ‘deduction of the supreme principle of morality’ (GMS 4: 463). Although notoriously obscure, this sketch shows that Kant hoped to base the moral law in the freedom that rational agents can claim as members of the ‘intelligible world’ that transcendental idealism makes available to us. In contrast, the second Critique abandons all aspirations of deriving morality from more basic notions of freedom and practical rationality.