From Supervenience to "Universal Law": How Kantian Ethics Become Heteronomous

In his Groundwork of the Metaphysics of Morals, Kant's desiderata for a supreme principle of practical reasoning and morality require that the subjective conditions under which some action is thought of as justified via some maxim be sufficient for judging the same action as justified by any agent in those conditions. This describes the kind of universalization conditions now known as moral supervenience. But when he specifies his "formula of universal law" (FUL) Kant replaces this condition with a quite different kind of universality: the judgment that some agent could rationally (i.e., without willing the frustration of his own valued ends) will his adoption of some maxim under the condition that this would cause all agents in his world to adopt it as well. Our wills typically lack this efficacy, so requiring that our wills conform to what would be rational for a hypothetical agent in this situation to will is a heteronomous requirement. Several intuitively wrong maxims pass Kant's test but fail the test of supervenience, because they generate no contradiction in a world of universal compliance but do so in non-ideal worlds, demonstrating the inadequacy of the FUL and the logical superiority of moral supervenience.