You can’t beat a kid’s head against the pavement just because he throws juice at you.

It was one of the first things I was told in nursery school.

Totally perplexing. Perhaps Mrs. Crossfield hadn’t heard: The. Kid. Threw. Juice. At. Me.

Years later and after countless trips to the principal’s office, the lesson sank in. A smart overhand lead to the jaw from Philip Pantana in the fourth grade clarified the concept.

We’re all responsible for our own actions. Two wrongs don’t make a right. Just because I flicked Philip in the back of the head with a finger, he didn’t have to clobber me. He did it by choice, just as I chose to lay on the ground for awhile and count the stars before getting back up.

Quakers have this elemental concept down pat. You can throw all the juice you want at a Quaker, in theory, and nothing will come back — not Welch’s, not Tropicana, certainly not a fist. By turning the other cheek to violence and affronts, a disciplined Quaker — a person of any or no faith, for that matter — accepts his actions wholly as his own. It’s Self Discipline 101, and it used to be better demonstrated in the public arena.

CNN reporter Jim Acosta’s and Donald Trump’s ignoble performances — note plural — at a recent White House news conference should have served to highlight this vanishing public virtue. Instead, the chattering class took up one side or the other largely: Trump was a jerk or Acosta was a jerk, depending on whom you like least. The truth is both acted poorly, and one’s misbehavior should have had nothing to do with the other’s in the country we strive to be.

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Acosta, who long ago surrendered journalism’s cardinal rule of covering the news and not becoming it, acted disrespectfully to a sitting U.S. president. He should own that. Trump lashed out at members of the White House press corps, belittling the presidency in the process. That belongs to him. There are no excuses for either; we all used to know that.

It must be difficult covering the Trump administration and not becoming changed as a journalist. White House press secretary Sarah Huckabee Sanders lies all the time, and the president goes out of his way to insult reporters. If he had said to me what he recently said to CNN’s Abby Phillip — “That’s a stupid question. But I watch you a lot, you ask a lot of stupid questions.” — I’d likely have fallen into the trap and been dragged away by the Secret Service.

But Phillip showed self-discipline and class. She kept her side of the street clean while a president of the United States dumped trash all over his own. And that’s really what he did.

It must be hard, too, working in Trump’s press staff. It’s all-out war between some news outlets and this administration, and every day must feel like Dien Bien Phu. A Pew Research Center study showed that only 5 percent of stories on the president in 2017 could be considered “positive,” and things don’t feel much more collegial now. If that weren’t bad enough, Trump administration officials have to watch for antifa types shouting at them in restaurants on the rare night out. That’s gotta stink.

We’re living under a presidency in which everyone feels extra justified in venting his or her spleen, but we seem to forget how that diminishes us individually and as institutions.

The Trump era will end soon enough. What will be left of us when it does? Who will we be?

William F.B. O’Reilly is a consultant to Republicans.