The Kantian approach to moral philosophy is to try to show that ethics is based on practical reason: that is, that our ethical judgments can be explained in terms of rational standards that apply directly to conduct or to deliberation. Part of the appeal of this approach lies in the way that it avoids certain sources of skepticism that some other approaches meet with inevitably. If ethically good action is simply rational action, we do not need to postulate special ethical properties in the world or faculties in the mind, in order to provide ethics with a foundation. But the Kantian approach gives rise to its own specific form of skepticism, skepticism about practical reason.

By skepticism about practical reason, I mean doubts about the extent to which human action is or could possibly be directed by reason. One form that such skepticism takes is doubt about the bearing of rational considerations on the activities of deliberation and choice; doubts, that is to say, about whether “formal” principles have any content and can give substantive guidance to choice and action. An example of this would be the common doubt about whether the contradiction tests associated with the first formulation of the categorical imperative succeed in ruling out anything. I will refer to this as content skepticism. A second form taken by skepticism about practical reason is doubt about the scope of reason as a motive.