My father had a drive toward being a cultured gentleman. I see, as I look back, that it was much more powerful than I thought as I was growing up, but even then, I noticed it. He was raised on a farm in eastern Pennsylvania as part of an old order community and when he chose to leave that community, he faced a bewildering array of options. I think the intensity of his focus on “being a gentleman” and on trying to make his four sons into gentlemen came from the great distance between what he left and what he chose. 
Part of Dad’s aspiration to gentility had to do with politics. Dad wasn’t oriented toward policy; he wanted presidential candidates who shared his concern for good manners and good language. Partly for that reason, President Truman was a difficulty for him. The trait that newscasters celebrated as “plain-spoken,” Dad saw as “vulgar.” One of the first political stories I remember from Dad was that “the dictionaries”—he may have had Webster’s 2nd Edition in mind—had been using the rule that a word will be added to the dictionary when it is used by the President. They had to stop using that rule, he said, when Truman became president.
This led in time to a joke that Washington “couldn’t tell a lie.” Roosevelt.couldn’t tell the truth. And Truman couldn’t tell the difference.”
It isn’t Dad’s conservatism I want to point to in remembering that story; it is the moral vacuity of the butt of the joke—Truman in this case.  And I got to remembering this when I readMichael Tomasky’s column in the New York Times this morning. Trump is the most egregious liar in the history of the presidency. Glenn Kessler of the Washington Post, who has been keeping track, has Mr. Trump at 15.6 lies a day, “roughly one every waking hour.”
Why does he do it?
As a (nearly) life long professor of political science, I used to field questions like this in class and the first thing I wanted to know from the questioner was, “Why do you want to know?” I would ask that because some reasons for wanting to know can be satisfied, even within the context of a political science class. Other reasons have no hope at all of being adequately addressed. In the case of President Trump, I have three answers in mind and none of them can be fully addressed by the social institutions we have now.
Three plausible reasons
The first follows from Dad’s joke about not being able to tell the difference. President Trump’s overwhelming interests, it seems to me, have to do with self-aggrandizement. Some say he is besotted by the quest for power, but it isn’t power to do something; it is power to avoid restraint.
Tomasky’s column notes that previous lying presidents have lied within the structure of existing political institutions. Even Nixon and George W. Bush recognized that there were other political institutions with their own legitimate powers and they needed to be dealt with in some way. Tomasky says that Trump doesn’t recognize any other legitimate powers at all. The center of moral worth is “the Presidency,” (meaning himself); before that, it was the campaign, (meaning himself); and before that it was his several businesses (meaning himself).
There is, in the President’s mind, a massive moral equivalency between “greatest” and himself. “Greatest” naturally inheres in himself. That is why the crowd at his inauguration is larger than any other crowd ever. It is why all Americans are safer and happier than they have ever been before. These are assertions that do not need confirmation—facts that bear on these assertions are irrelevant to him. “Facts” are just tools to support a “truth” that is obvious to him, which is that he and his are the greatest. So assertions of “fact” are not, principally, true or not; they are useful or not.
The heart of President Trump’s lying, in this view, is that “truth” has no merit at all apart from utility. He asserts that it is true if it is useful. 
The second reason he lies is to defame his enemies. The core of President Trump’s base is angry at the way they have been treated. The Trump style of campaigning not only defames these enemies, but makes fun of them. He says they are bad, in other words, and also makes them objects of derision. I love the idea of “leaving the sociopath,” but it will require winning a very important election to do that.
This is a separate reason for lying. It has no direct connection to the Trump fetish about being the greatest. This is giving “talking points” to people who was to “hit back.” These people are aggrieved, remember, and whatever they do, is something “back.” They are “retaliating.” Notice the re- in retaliating; It represents the “back” in “hitting back.” And not only does it give talking points, it gives permission to say things like that. These are social slurs or ethnic slurs or class slurs. These are things that until recently, were not OK to say in public. The avalanche of Trump lies addresses these two problems: it justifies language that used to be “bad manners” and it scripts the charges against their enemies. And…of course…their truth of falsity is not an obstacle. Not for a man who tells 15.6 lies a day.
The third reason is that it puts the news media in an awful spot. The game the media have been playing has been the fact game and that game has been further inflamed by the “both sides of the story” game. President Trump’s drumbeat of outrageous lies causes the media to fail at both of the games they are accustomed to playing and that is another reason, as I see it, that he lies so much.
The match President Trump wins by lying in ways the media cannot afford to pass unchallenged is the match of the narrative against the facts. President Trump’s narrative presumes a factual basis, although it is false. The media can demolish the factual claims one by one but the revelation that the facts are fraudulent doesn’t damage the narrative. The both sides of the story game requires the media to give equal weight to the most sober investigation and the most transparent lies on the grounds that they represent two “sides.”
The most recent response by the press is to aggressively call President Trump’s lies for what they are. This doesn’t work either. This is equivalent to the referee starting a fight with a pitcher who threw a beanball or with a defensive end who laid a late hit on the quarterback. The referee cannot become a participant and still adjudicate quarrels between players. The New York Times cannot challenge the Trump administrations claims as intentional and unconscionable lies without being “an opposing player.” The guy in the yellow shirt, no matter how severely he was provoked, is no longer refereeing the game.
So in response to the lies of the Trump administration, the media has three options, all of which set the President up to win. And that is the third reason he lies so much.
President Trump’s lying is, in other words, overdetermined. Any of the three reasons for lying consistently is adequate to maintain the pattern.
I am very much encouraged, myself, by the fact that some lies are federal offenses. He won’t win that one.
 There was still a lot of distance to cover for the sons, because being the children of a father who grew up on a farm in an old order community was an identity we had to cope with.
 It’s really not a bad joke. It switches the meaning of the verb “tell” in the middle of the joke, from “speak to “distinguish,” so that “tell” in the last use means something different that it did in the two previous uses.
 Try to imagine designing a lie detector test this man could not pass.