Our ability to know and act in accord with the divine law was denied by the voluntarism of the Protestant Reformers and their acceptance of a strong doctrine of original sin, while the teleological conception of nature at the heart of the classical conception of human life was abandoned with the acceptance of the new mechanistic science. With these classical props for the moral rules no longer available, it was inevitable that some alternative structure for justifying the moral rules should be sought, and the Humean and Kantian constructions are the fruits of this search.

 [Alasdair] MacIntyre argues that these Enlightenment responses had to fail precisely because the project of the Enlightenment with regard to the justification of moral rules was incoherent. The moral rules that the Enlightenment theories aimed to justify were crafted as corrective devices for human nature as we find it. They arose as devices for perfecting human beings whose natural state is imperfect in several ways. The perfective process (whether in Aristotle’s pagan view or Aquinas’s Christian one) involved a movement toward an idealized end of human life – toward the human good. It was only within a broadly teleological conception of human life that such a conception of the moral good was coherent. In the absence of some rich notion of perfected humanity – a notion underwritten by classical teleology and the notion of divine law – the only remaining basis for grounding the moral rules are features of human nature in its unperfected state.