In “The Internet of Us,” Michael P. Lynch sees threats to our capacity to reason. Illustration by Matthieu Bourel

Ted Cruz’s campaign autobiography is called “A Time for Truth.” “This guy’s a liar,” Donald Trump said at a recent G.O.P. debate, pointing at Cruz. Trump thinks a lot of people are liars, especially politicians (Jeb Bush: “Lying on campaign trail!”) and reporters (“Too bad dopey @megynkelly lies!”). Not for nothing has he been called the Human Lie Detector. And not for nothing has he been called a big, fat Pinocchio with his pants on fire by the fact-checking teams at the Times, the Washington Post, and Politifact, whose careful reports apparently have little influence on the electorate, because, as a writer for Politico admitted, “Nobody but political fanatics pays much mind to them.” “You lied,” Marco Rubio said to Trump during the truth-for-tat February debate. Cruz tried to break in, noting that Rubio had called him a liar, too. Honestly, there was so much loudmouthed soothsaying that it was hard to tell who was saying what. A line from the transcript released by CNN reads:

UNIDENTIFIED MALE: I tell the truth, I tell the truth.

Eat your heart out, Samuel Beckett.

On the one hand, not much of this is new. “Gen. Jackson is incapable of deception,” Andrew Jackson’s supporters insisted, in 1824. “Among all classes in Illinois the sobriquet of ‘honest Abe’ is habitually used by the masses,” a Republican newspaper reported of Lincoln, in 1860. The tweets at #DumpTrump—“This man is a hoax!”—don’t quite rise to the prose standard of the arrows flung at supporters of John Adams, who Jeffersonians said engaged in “every species of villainous deception, of which the human heart, in its last stage of depravity is capable.”

“When a President doesn’t tell the truth, how can we trust him to lead?” a Mitt Romney ad asked last time around, during an election season in which the Obama campaign assembled a so-called Truth Team to point out Romney’s misstatements of fact. Remember the Swift Boat Veterans for Truth, from 2004? This kind of thing comes and goes, and, then again, it comes. Cast back to Nixon: Among all classes, the sobriquet of “Tricky Dick” was habitually used by the masses. “Liar” isn’t what opponents generally called Ford or Carter or the first George Bush, but a Bob Dole ad, in 1996, charged that “Bill Clinton is an unusually good liar,” and much the same was said of Hillary Clinton, dubbed “a congenital liar” by William Safire. A Bernie Sanders campaign ad refers to him, pointedly, as “an honest leader”; his supporters have been less restrained. At a rally in Iowa, they chanted, “She’s a liar!”

On the other hand, some of this actually is new. When a sitting member of Congress called out “You lie!” during the President’s remarks before a joint session in 2009, that, for instance, was new. (“That’s not true,” Obama replied.) John Oliver’s #MakeDonaldDrumpfAgain campaign is both peerless and unprecedented. On HBO, Oliver checked Trump’s facts, called Trump a “litigious serial liar,” and dared him to sue. Also newish is the rhetoric of unreality, the insistence, chiefly by Democrats, that some politicians are incapable of perceiving the truth because they have an epistemological deficit: they no longer believe in evidence, or even in objective reality.

To describe this phenomenon, Democrats go very often to the Orwellian well: “The past was erased, the erasure was forgotten, the lie became truth.” Hillary Clinton has a campaign ad called “Stand for Reality.” “I’m just a grandmother with two eyes and a brain,” she says, which is an awfully strange thing for a former First Lady, U.S. senator, and Secretary of State to say. But what she means, I guess, is that even some random old lady can see what Republican aspirants for the Oval Office can’t: “It’s hard to believe there are people running for President who still refuse to accept the settled science of climate change.”

The past has not been erased, its erasure has not been forgotten, the lie has not become truth. But the past of proof is strange and, on its uncertain future, much in public life turns. In the end, it comes down to this: the history of truth is cockamamie, and lately it’s been getting cockamamier.

Most of what is written about truth is the work of philosophers, who explain their ideas by telling little stories about experiments they conduct in their heads, like the time Descartes tried to convince himself that he didn’t exist, and found that he couldn’t, thereby proving that he did. Michael P. Lynch is a philosopher of truth. His fascinating new book, “The Internet of Us: Knowing More and Understanding Less in the Age of Big Data,” begins with a thought experiment: “Imagine a society where smartphones are miniaturized and hooked directly into a person’s brain.” As thought experiments go, this one isn’t much of a stretch. (“Eventually, you’ll have an implant,” Google’s Larry Page has promised, “where if you think about a fact it will just tell you the answer.”) Now imagine that, after living with these implants for generations, people grow to rely on them, to know what they know and forget how people used to learn—by observation, inquiry, and reason. Then picture this: overnight, an environmental disaster destroys so much of the planet’s electronic-communications grid that everyone’s implant crashes. It would be, Lynch says, as if the whole world had suddenly gone blind. There would be no immediate basis on which to establish the truth of a fact. No one would really know anything anymore, because no one would know how to know. I Google, therefore I am not.

Lynch thinks we are frighteningly close to this point: blind to proof, no longer able to know. After all, we’re already no longer able to agree about how to know. (See: climate change, above.) Lynch isn’t terribly interested in how we got here. He begins at the arrival gate. But altering the flight plan would seem to require going back to the gate of departure.

Historians don’t rely on thought experiments to explain their ideas, but they do like little stories. When I was eight or nine years old, a rotten kid down the street stole my baseball bat, a Louisville Slugger that I’d bought with money I’d earned delivering newspapers, and on whose barrel I’d painted my last name with my mother’s nail polish, peach-plum pink. “Give it back,” I told that kid when I stomped over to his house, where I found him practicing his swing in the back yard. “Nope,” he said. “It’s mine.” Ha, I scoffed. “Oh, yeah? Then why does it have my name on it?” Here he got wily. He said that my last name was also the name of his baseball team in the town in Italy that he was from, and that everyone there had bats like this. It was a dumb story. “You’re a liar,” I pointed out. “It’s mine.” “Prove it,” he said, poking me in the chest with the bat.