Kantian Naturalism in Moral Theory

Introducing the notion of «cosmic [or cosmopolitan] concept» (Weltbegriff) of philosophy in the Critique of Pure Reason, Kant explains that «In this view philosophy is the science of the relation of all cognition to the ultimate and essential aims of human reason», and a few lines later declares that «In view of the complete systematic unity of reason, there can only be one ultimate end of all the operations of the mind. […] This ultimate end is the destination of man, and the philosophy which relates to it is termed moral philosophy» (KrV B 867-868 trans. J.M.D. Meiklejohn). Kant concludes that «The legislation of human reason, or philosophy, has two objects – nature and freedom – and thus contains not only the laws of nature, but also those of ethics, at first in two separate systems, which, finally, merge into one grand philosophical system of cognition. The philosophy of nature relates to that which is, that of ethics to that which ought to be» (ibidem). The strong emphasis on the unity of cognition means that, in the end, philosophy of nature and ethics are part of a unified system, where nature and freedom are reconciled from the perspective of the destination of man. In this perspective, one could expect that the foundations of morality, in such a final system, are not totally isolated from the foundations of nature.
Yet in what sense, if any, can a Kantian perspective appeal to the notion of nature as a foundation for moral normativity? An answer to this question is better understood as a challenge for ethical theory rather than an exegetical problem, but there are grounds for suggesting that Kant’s notion of nature admits of such a possibility, notwithstanding his well known attacks on attempts at grounding morality in (a certain meaning of) nature.