The recent literature contains a number of relatively successful Kantian responses to the familiar objection, usually associated with Hegel, that the categorical imperative is “empty” in the sense that, of itself, it is incapable of either generating any specific duties or ruling out any maxims as impermissible. Although these responses hardly resolve all of the difficulties regarding the nature and applicability of the categorical imperative, they do show that the emptiness charge rests on a serious misunderstanding of Kant's theory, including a failure to attend to the way in which Kant construes the “contradiction in conception” and “contradiction in will” tests.

These responses fail, however, to address a related but distinct difficulty in the derivation of the categorical imperative that is treated at some length by Bruce Aune in Kant's Theory of Morals. This difficulty concerns an apparent gap in Kant's move from the principle that one's actions must conform to universal law, which Aune regards as a non-problematic requirement of rational willing, to the categorical imperative in its canonical universal law formula. Since, according to Aune, the difference between the requirement of conformity to universal law and the categorical imperative consists in the fact that the former does not provide a guide to action, whereas the latter presumably does, this criticism leads to the same conclusion as the emptiness charge: Kant fails to establish a viable, contentful principle of moral discrimination.