Taking metaphysics seriously: Kant on the foundations of ethics

Ask most philosophers for an example of a moral rationalist, and they will probably answer “Kant.” And no wonder. Kant's first great work of moral philosophy, Groundwork of the Metaphysics of Morals , opens with a clarion call for rationalism, proclaiming the need to work out for once a pure moral philosophy, a metaphysics of morals. That this metaphysics includes the first principle of ethics, the moral law, is obvious. But what about the second principles, particular moral laws, such as duties of truthfulness, beneficence, etc.? Are these principles metaphysical too? Many have thought not, since they make essential use of empirical and anthropological considerations. I argue otherwise; the second principles are metaphysical and this fact matters. I do this by taking seriously the metaphysics of the metaphysics of morals—more specifically, by understanding the metaphysics of morals alongside the metaphysics of nature. For, qua metaphysics, both employ a common two‐stage methodology, the first stage of which is wholly a priori but the second stage of which is partly empirical. As I explain, appreciating this common methodology sheds new light on how the second principles are to be established, as well as on the reach of Kant's rationalism.