Republican presidential candidate Donald Trump gives a thumbs up after giving a foreign policy speech at the Mayflower Hotel in Washington on April 27. (Evan Vucci/Associated Press)

Danielle Allen is a political theorist at Harvard University and a contributing columnist for The Post.

Many things have made me sad during the long and difficult months of our presidential campaign, so many that it scarcely feels worth enumerating them. Still, for all that we have had to endure, now and then there comes something so low as to require comment. I’d like to pause to consider what has become of the word “presidential.”

Everybody appears to agree that Donald Trump needs to become, or “appear,” more “presidential.” The chorus includes the media, Melania and Ivanka Trump, Trump adviser Paul Manafort and even — at least at times — the candidate himself. Everybody also seems to agree on what being presidential requires: fewer swear words, fewer insults, a tone of decorum, speechwriters and teleprompters.

How sad. All of that is cosmetic. Trump is, of course, an expert in cosmetics. He has a fragrance line, and his wife and daughter market beauty products. He has demoted the art of presidential leadership to skill at cosmetology.

America, do you remember what presidential leadership ought properly to consist of? Linger with me for a moment on the question.

Once upon a time, presidential leadership was about not appearances but statesmanship. So what is statesmanship? The Declaration of Independence begins with the phrase “When in the course of human events.” The committee that drafted that text liked the phrase “course of human events” because it conjures up the image of a waterway, a river or sea that must be navigated. The 17th-century political philosopher John Locke also used the image of a ship at sea to describe the problem of politics.

The statesman is like the helmsman. First, he or she must understand choppy waters, see stormy weather on the horizon, and chart and navigate currents flowing deep beneath the surface. Human life is like this, too, a welter of swirling detail in which we must discern patterns, the directions that events are tending. Our capacities for discernment lead us to see the choices that we confront.

Trump puts on a good performance of plotting out where we are heading, hence his insistent messages about trade, immigration and the Islamic State. But his diagnoses are wrong. We do have work to do to adjust trade policy to protect laborers in the United States, but closing our borders will not bring us prosperity. Similarly, building a wall along the Mexican border will not halt the flow of drugs into this country, which also enter through Canada and come by sea, not to mention through tunnels under our existing border barriers and steadily increasing domestic production. Pausing entry into the country by Muslims will leave us no basis for the cooperative relationships that we need to defeat the Islamic State. Trump is playing a part, grandiosely spreading out mock charts of the seas before us. But he is play-acting, America. He is not, in fact, diagnosing our circumstances accurately.

Second, the statesman or helmsman must know not only how to read the seas but also how to set a direction for us. Where do we want to go? We need our North Star. Here we need principles. Here is where Trump falls farthest short. He has no principles other than protection of his tribe, his family, people who look like his family and fans who at some level yearn to have or be part of his life.

We know he has no principles because of something that he said in his foreign policy speech last week. It was extremely odd. Here is the passage:

“I will work with our allies to reinvigorate Western values and institutions. Instead of trying to spread universal values that not everybody shares or wants, we should understand that strengthening and promoting Western civilization and its accomplishments will do more to inspire positive reforms around the world than military interventions.”

Here Trump contrasts Western values and universal values. This is bizarre in the extreme. The pride and glory of the Western tradition is the Enlightenment, also known as the Age of Reason, which brought us a belief in universal reason and the universal rights of human beings, and in these gave us the foundations of the democratic republics we now inhabit.

Or take the Western tradition all the way back to ancient Greece, if you want. Plato defended Truth, with a capital T, as a universal value. This is a value that Trump knoweth not.

Trump is trying to convince us that the Western tradition amounts to no more than a set of ethnic habits, and the habits of white ethnics, at that. Plato, Aristotle, Locke, Kant, Hume, Smith, Rousseau, Adams, Jefferson and Washington are rolling in their graves. All aspired to something much bigger than tribalism.

But I digress. My point is that statesmanship consists of joining accurate diagnoses of our circumstances to morally compelling accounts of where we should go. Trump currently earns a B-minus on the former and an F on the latter. Becoming presidential would require him to fix these two problems.

So, America, please get a grip. Please take on board the difference between cosmetology and statesmanship. I admire my hairdressers. Each and every one of mine is a good friend. And I believe that any hairdresser can also be a statesman. But the particular hairdresser we have in front of us on the national stage doesn’t have any but the shadowiest of ideas of what statesmanship requires.