The Project of Resolving Moral Issues by Liberal Argumentation Has Failed

"For the subsequent history of moral philosophy has been a history of ramifying disagreement in which all of [Henry] Sidgwick's attempted reconciliations of hitherto warring post-Enlightenment points of view into a synthesis -- which was itself intended to foreshadow a coming convergence of an even more complete kind -- I have been dissolved into new and multifarious conflicts. Universalizability theorists, utilitarians, existentialists, contractarians, those who assert the possibility of deriving morality from rational self-interest and those who deny it, those who uphold the overriding character of an interpersonal standpoint and those who insist upon the prerogatives of the self, disagree not only with each other but among themselves, and the certitude of those who maintain each point of view is matched only by their inability to produce rational arguments capable of securing agreement from their adversaries. Thus post-Sidgwickian moral philosophy, judged by the standards of... Sedgwick himself, has turned out to be a dubious type of activity, self-discrediting in just the way that Sedgwick held that the theology of the late nineteenth century was self-discrediting." -- Alasdair MacIntyre, Three Rival Versions of Moral Enquiry, p. 189

The liberal, Enlightenment project was to construct a morality, by "pure reason" alone, without assuming any particular metaphysical stance, to which all rational people could agree. In the late nineteenth century, when Sidgwick wrote the entry on "Ethics" for the Ninth Edition of the Encyclopaedia Britannica, liberal ethicists were largely convinced that they were "almost there," and all that was left was a mopping up operation. Their conviction has been spectacularly falsified by the century plus since then.

And we can understand why their project had to fail: different metaphysical and theological standpoints imply different bases for ethical reasoning, and thus different conclusions. Any method of proceeding that declares these different bases off limits for "reasonable" discussion is inherently incapable of reconciling those differing conclusions.