Donald Trump's presidency is a disaster. But it's a disaster that may teach liberals a valuable moral lesson.

Most liberals tend to view morality in horizontal terms — as a matter of securing and protecting the rights of free and equal individuals. This leads liberals to emphasize procedures and processes that ensure fairness for all. Many other liberals, especially those primarily concerned about issues wrapped up with identity politics, highlight another horizontal aspect of morality: recognition of the value and worth of different groups. This goes beyond equal rights to demand that fellow citizens and the government itself actively affirm the goodness of different ways of life. People don't merely have rights that give them the freedom to live as they wish. They actually deserve to enjoy positive affirmation in the public square.

Leaving aside the very real and important conflicts between these two constellations of liberal moral concern, they both presume and seek to enforce equality or egalitarianism. That's why I've described them as fostering a horizontal vision of morality.

But egalitarianism doesn't exhaust moral experience. On the contrary, morality also has a different dimension — one having to do not with equality but with inequality, distinction, nobility, elevation, sanctity, excellence, and virtue. This vertical aspect of morality is absolutely crucial for understanding politics, but liberals tend to neglect it — or at least took it for granted until Trump became president.

Trump morally offends liberals in many ways. A number of them have to do with horizontal concerns: offenses against the rights of various individuals and groups, such as the poor, minorities, immigrants, and Muslims. The liberal response to these offenses is to reaffirm the transgressed rights and attack the president for his divisiveness, cruelty, and failure to affirm equality for all.

Then there are Trump's myriad offenses against the rule of law. In response to these, liberals reaffirm the principle and insist that the president be held to the same exacting standards that have applied to his predecessors. Again, equality is the norm and the measure, across presidential administrations over time.

But there's another way that Trump offends liberals (as well as many on the center-right): with his angry and insulting tweets, attacks on the press, and continuous stream of lies. What makes this behavior so bad? Charles Blow of The New York Times spoke for many in a recent column that used a series of terms one now regularly hears tripping from liberal lips: "We must remind ourselves that Trump's very presence in the White House defiles it and the institution of the presidency. Rather than rising to the honor of the office, Trump has lowered the office with his whiny, fragile, vindictive pettiness."

Every italicized word is a term of distinction, referring to and presuming the possibility of making vertical moral distinctions: pure and defiled, rising and falling, honorable and dishonorable, higher and lower. The same kind of distinctions are implied every time someone describes Trump's actions or statements as "unpresidential." Since he's the president, the claim would seem to be self-refuting — unless, that is, we believe that the office of the presidency itself, apart from the behavior of any particular president, is honorable, noble, elevated, exalted, something to which we rightly look up and from which a particular president can diverge or fall short.

There are many ways to conceive of and think about such vertical moral distinctions. Aristotle treated them as woven into the fabric of political life — and because human beings are political animals, he also assumed they were woven into the fabric of human life, where they could be studied to teach us crucially important lessons about the longings that most powerfully move the human soul. Meanwhile, the political-theological traditions within Judeo-Christianity appeal to the God divinely revealed in the Bible to explain, limit, and complete these vertical moral intuitions. In our time, social psychologist Jonathan Haidt has devised his own empirically grounded theory of moral foundations that gives vertical moral distinctions their due.

The liberal political tradition from Hobbes to Rawls, by contrast, has always been suspicious of the vertical dimension of morality, worrying that it fosters aristocratic and illiberal passions that decent politics must constrain, thwart, or channel into less publicly dangerous pursuits. Yet the most thoughtful liberal thinkers have also understood that decent politics necessarily presupposes that citizens affirm the reality of such distinctions.

For those liberals inclined to forget the need for them, or to complacently assume that they will always be there to draw on and elevate public life, the jarring experience of living under President Trump is a potent reminder of just how crucially important (and fragile) vertical moral distinctions really are. It also demonstrates that transgressions of vertical ideals can feel just as wrong — just as much a violation of an intrinsic standard of right — as transgressions against horizontal-egalitarian notions of equal dignity before the law.

If the nation's bracing experience of life with a profoundly unpresidential president manages to open liberals to the importance of the vertical dimension of morality, it will have had at least one positive result.