WASHINGTON — Samuel Johnson has James Boswell. Lyndon Johnson has Robert Caro. Donald Trump has Glenn Kessler of The Washington Post, but Kessler’s more nemesis than amanuensis.

Every untruth Trump utters, Kessler chronicles. At the beginning of this month, the running total was 4,229 false or misleading statements since the president took office, or more than 7.5 a day. This will be remembered. Historians will examine how an American presidency parted company with facts, and will assess the toll.

Kessler is a dapper, mild-mannered guy who says he’s “pretty even-keeled,” even as Trump messes hourly with his head. The son of Dutch immigrants, he meets me for lunch with his right hand bandaged from a collision with a wine glass while he was washing the dishes. He covered “every building in D.C.,” as he puts it, before editors persuaded him seven years ago to write “The Fact Checker,” awarding “Pinocchios” on a scale of one (for the shading of facts) to four (for a whopper). Over the years, a Pinocchio has entered the Washington political lexicon as a unit of dishonesty. Now it defines the zeitgeist.

Trump is well known to Kessler. As a reporter at Newsday, Kessler covered Trump’s real estate business shenanigans, finding him “boastful” and given to “laxity about the truth.” Banks tended not to trumpet Trump’s falsehoods; they just declined further loans. What has changed is his world more than his behavior.