During the 1760s and 1770s, Kant entertained a naturalistic approach to ethics based on the supposed psychological fact of a human love for freedom. During the critical period, especially in the Groundwork for the Metaphysics of Morals, Kant clearly rejected such an approach. But his attempt at a metaphysical foundation for ethics in section III of the Groundwork was equally clearly a failure. Kant recognized this in his appeal to the "fact of reason" argument in the Critique of Practical Reason, but thereby gave up on any attempt to ground the fundamental principle of morality at all. So it is of interest to see how far we might now proceed along the lines of his original naturalistic approach.