This thesis considers Kant's departure from moral naturalism. In doing so, it explores the relationship between ethics, naturalism, normativity and freedom. Throughout this exploration, I build the case that Kant's ethics of autonomy allows us to make better sense of ethics than Hume's moral naturalism. Hume believes that morality is ultimately grounded in human nature. Kant finds this understanding of ethics limiting. He insists that we are free - we can critically reflect upon our nature and (to an extent) alter it accordingly. This freedom, I contend, renders the moral naturalist's appeal to nature lacking. Of course, a Kantian conception of freedom - some form of independence from the causal order - is fairly unpopular in contemporary circles. In particular, a commitment to naturalism casts doubt on such a notion of freedom. I argue with Kant that such a conception of freedom is essential to the conception of ourselves as rational agents. The critical turn, unlike naturalism, warrants this conception of freedom, accommodating the point of view of our rational agency. It thus allows Kant's ethics of autonomy to better grasp certain key elements of morality - normativity and our agency - than Hume's moral naturalism.