David Priess, an intelligence officer who once gave presidential daily briefings, asked me whether I thought Mr. Trump could distinguish between truth and untruth. He raised the controversial speech Mr. Trump gave at a Boy Scout Jamboree in West Virginia in July 2017, a speech that was overly political and occasionally tasteless. In the face of sharp criticism, the president said that the Scouts’ leader had called him to say it was “the greatest speech that was ever made to them.”

Of course, no such call ever occurred. But was Mr. Trump actually able to draw a distinction between the past that had really happened and the past that he needed at that moment? Mr. Priess’s point was that you could sometimes convince a liar that he was wrong. What do you do with someone who does not distinguish between truth and untruth?

We in the intelligence world have dealt with obstinate and argumentative presidents through the years. But we have never served a president for whom ground truth really doesn’t matter.