Alasdair MacIntyre, a philosopher who metamorphosed from a London Marxist into a Midwestern American Catholic during a decades-long quest to prove there was an objective foundation to moral virtue — a lonely project that struck many of his academic peers as anachronistic yet drew a large, varied and growing crowd of admirers — died on May 21. He was 96.

His death was announced by the University of Notre Dame, where Mr. MacIntyre was a professor emeritus of philosophy. The announcement did not say where he died.

Moral beliefs are widely considered matters of private conscience — up for debate, of course, but not resolvable in any sort of final consensus. That is why, for example, people generally think teachers should guide students toward self-realization, rather than proselytize their own beliefs. The same neutrality is expected of lawyers, therapists, government officials and others.

Mr. MacIntyre belonged to a different moral universe.

In his best-known book, “After Virtue” (1981), he argued that thousands of years ago, the earliest Western philosophers and the Homeric myths generated “the tradition of the virtues,” which was treated as objective truth. Value neutrality, to Mr. MacIntyre, was the goal of “barbarians” and a sign of “the new dark ages which are already upon us.”

Such language might make Mr. MacIntyre seem like a wistful reactionary. In fact, his worldview was far less predictable.

He never entirely disavowed his youthful Marxism, applauding Marx’s critique of the individualistic and acquisitive spirit of capitalism. He maintained a certain sort of modesty from his days as a self-appointed champion of the working class — he never earned a Ph.D. and disliked being called “professor” — and he continued showing the dialectical passion of a Trotskyist, occasionally launching into what one colleague called “MacIntyrades.”