Within Kantian ethics and Kant scholarship, it is widely assumed that autonomy consists in the self-legislation of the principle of morality. In this paper, we challenge this view on both textual and philosophical grounds. We argue that Kant never unequivocally claims that the Moral Law is self-legislated and that he is not philosophically committed to this claim by his overall conception of morality. Instead, the idea of autonomy concerns only substantive moral laws, such as the law that one ought not to lie. We argue that autonomy, thus understood, does not have the paradoxical features widely associated with it. Rather, our account highlights a theoretical option that has been neglected in the current debate on whether Kant is best interpreted as a realist or a constructivist, namely that the Moral Law is an a priori principle of pure practical reason that neither requires nor admits of being grounded in anything else.