MacIntyre on Justifying Morality
This passage from After Virtue remains the most illuminating thing I have ever read on moral philosophy:
Consider … the general form of the moral scheme which … came for long periods to dominate the European Middle Ages from the twelfth century onwards, a scheme which included both classical and theistic elements. Its basic structure is that which Aristotle analyzed in the Nicomachean Ethics. Within that teleological scheme there is a fundamental contrast between man-as-he-happens-to-be and man-as-he-could-be-if-he-realized-his-essential-nature. Ethics is the science which is to enable men to understand how they make the transition from the former state to the latter. Ethics therefore in this view presupposes some account of potentiality and act, some account of the essence of man as a rational animal and above all some account of the human telos. The precepts which enjoin the various virtues and prohibit the vices which are their counterparts instruct us how to move from potentiality to act, how to realize our true nature and to reach our true end. To defy them will be to be frustrated and incomplete, to fail to achieve that good of rational happiness which it is peculiarly ours as a species to pursue. The desires and emotions which we possess are to be put in order and educated by the use of such precepts and by the cultivation of those habits of action which the study of ethics prescribes; reason instructs us both as to what our true end is and as to how to reach it. We thus have a threefold scheme in which human-nature-as-it-happens-to-be (human nature in its untutored state) is initially discrepant and discordant with the precepts of ethics and needs to be transformed by the instruction of practical reason and experience into human-nature-as-it-could-be-if-it-realized-its-telos. Each of the three elements of the scheme—the conception of untutored human nature, the conception of the precepts of rational ethics and the conception of human-nature-as-it-could-be-if-it-realized-its-telos—requires reference to the other two if its status and function are to be intelligible.
That’s on pp. 52–3 of the third edition. The chapter is called “Why the Enlightenment Project of Justifying Morality Had to Fail,” the point being that the second element in the threefold scheme has disappeared.