We get the word “mesmerize” from a doctor named Franz Anton Mesmer, who in Paris in the late 18th century posited the existence of an invisible natural force connecting all living things — a force you could manipulate to physically affect another person.

Mesmer’s work inspired the stage hypnotists of mid-19th-century America. Before rapt audiences, these “mesmerists” used carefully choreographed gestures to lull their subjects into a state of credulous obedience. As the practitioner John Bovee Dods wrote in the 1850s, a mesmerist could make an entranced volunteer see “that a hat is a halibut or flounder; a handkerchief is a bird, child or rabbit; or that the moon or a star falls on a person in the audience, and sets him on fire.”

In our historical moment, the mesmerists are worth considering, for they were frequently debunked but the debunkings rarely had much of an effect. Just as the repeated corrections of President Trump’s falsehoods have failed to discourage him or his supporters, so too the mesmerists escaped their exposés unharmed….

Like the mesmerists, what Mr. Trump is actually selling is anti-mesmerism. The mesmerists were offering a fantasy of turning the tables on con men by exposing their tricks. Mr. Trump, while constantly lying, denounces liars all around him. He tells us the game is rigged. The fake news media can’t out-fake him. This is how he gets away with making myriad demonstrably inaccurate claims, as he did at a rally with adoring supporters in Pennsylvania on Thursday, while in the same breath attacking the news mediaas “fake, fake disgusting news.”

Even if Mr. Trump’s audience retains a suspicion that he himself might be a swindler, that doesn’t necessarily work to his disadvantage. The attacks on his truthfulness have an alarming tendency to reinforce his message that he’s a master of the deceptive arts. In a treacherous world, you need a treacherous ally — treacherous, at least, to your mutual enemies. So become Mr. Trump’s apprentice! Sign up, as Dods urged, for the private class!

When no one is trustworthy, you might as well trust a con artist. There’s a strange logic to the idea. Innocent lambs may be admirable, but they’re not the defenders you want in a dog-eat-dog world. Better to have a sly fox at your side. ...