Are the bonds of American national unity becoming dangerously frayed? To judge from a steady stream of books, op-ed pieces, opinion journal articles, and talk-show conversations on this matter, the disturbing answer is yes.

Consider one example among many: A new book argues that there are within our borders four distinct “Americas,” each competing for recognition as the authentic America. But why only four? Why not five or six? Is there a prize for the contestant who argues credibly for the highest number?

Making allowance for the herd instinct of the chattering class, however, there really is a spirit of division abroad in today’s America. But it goes back further and reaches deeper than the pundits seem to grasp.

It became widely visible in the 1960s — the age of pot, Vietnam, sexual revolution, and campus unrest — but its underlying causes began much earlier. Basically, it’s a split between a traditional morality that grounds rights in duties that arise from our shared human nature and a new morality of individualistic self-gratification.

That new morality found iconic expression in the Supreme Court’s 1992 decision in Planned Parenthood v. Casey ratifying the right to abortion. The “heart of liberty,” proclaimed Justices Kennedy, O’Connor, and Souter, is “the right to define one’s own concept of existence, of meaning, of the universe, and of the mystery of human life.”

Previously, I supposed that the universe was as it was, not as I wanted it to be, but here was a higher wisdom.

And where did it start? For answers, turn to two thinkers: theologian Father John Courtney Murray, SJ, and Notre Dame philosopher Alasdair MacIntyre.

In his famous book “We Hold These Truths,” published on the eve of the 1960s cultural revolution, Father Murray located the source of emergent civil disharmony in the collapse of the natural law tradition with its system of values based upon shared human nature.

“Those who seek the ironies of history,” he wrote sadly, “should find one here, in the fact that the ethic which launched Western constitutionalism and endured long enough as a popular heritage to give essential form to the American system of government has now ceased to sustain the structure and direct the action of this constitutional commonwealth.”

MacIntyre, writing two decades later in “After Virtue,” pointed to the obvious fact that agreement on fundamental rules is required for social peace. Hence the social calamity resulting from general repudiation of the old consensus grounded in the Aristotelian-Thomistic tradition of moral reasoning (natural law again).

No mere fad of the chattering class, this has serious practical consequences. “Modern society,” he wrote, “is indeed often, at least in surface appearances, nothing but a collection of strangers, each pursuing his or her interests under minimal constraints.” In a fractured society like this, conflicts cannot be resolved by appeals to shared fundamental rules, since the old consensus on rules no longer exists. Doubt that? Observe the unhealthy practice now all too common of making, or attempting to make, political statements via mob action. It is as MacIntyre memorably put it: “Modern politics is civil war carried on by other means.”

The need for a restored national consensus embodying healthy shared values is large and growing. A political party or leader capable of articulating one would be rendering a profound national service. But whether any of our present contenders can rise to the occasion or even seriously aspires to do so is far from clear. One thing for sure: Chatter about national (dis)unity will continue.