Welcome to — brace yourself for — the post-truth presidency.

“Facts are stubborn things,” said John Adams in 1770, defending British soldiers accused in the Boston Massacre, “and whatever may be our wishes, our inclinations, or the dictates of our passion, they cannot alter the state of facts and evidence.”

Or so we thought, until we elected to the presidency a man consistently heedless of truth and impervious to fact-checking.

Oxford Dictionaries last month selected post-truth — “relating to or denoting circumstances in which objective facts are less influential in shaping public opinion than appeals to emotion and personal belief” — as the international word of the year, and for good reason.

The practice of post-truth — untrue assertion piled on untrue assertion — helped get Donald Trump to the White House. The more untruths he told, the more supporters rewarded him for, as they saw it, telling it like it is.

As Politico’s Susan Glasser wrote in a sobering assessment of election coverage for the Brookings Institution, “Even fact-checking perhaps the most untruthful candidate of our lifetime didn’t work; the more news outlets did it, the less the facts resonated.”

Indeed, Hannah Arendt, writing in 1967, presciently explained the basis for this phenomenon: “Since the liar is free to fashion his ‘facts’ to fit the profit and pleasure, or even the mere expectations, of his audience, the chances are that he will be more persuasive than the truth teller.”

So there is no reason to think Trump is about to suddenly truth-up. Indeed, all signs are to the contrary — most glaringly Trump’s chockfull-of-lies tweet that “I won the popular vote if you deduct the millions of people who voted illegally.”

Trump and his aides are not embarrassed by their post-truthism — they embrace it. Three data points from last week:

First, quasi-fired Trump campaign manager Corey Lewandowski at Harvard University, making the astonishing assertion that the media’s failing during the campaign was not that it scorned Trump — it was that it believed him.

“You guys took everything that Donald Trump said so literally,” he said. “The American people didn’t. They understood it. They understood that sometimes — when you have a conversation with people . . . you’re going to say things, and sometimes you don’t have all the facts to back it up.”

Second, this eye-popping assertion from Trump supporter/CNN commentator Scottie Nell Hughes on the Diane Rehm Show: “People that say that facts are facts — they’re not really facts . . . there’s no such thing, unfortunately, anymore of facts. And so Mr. Trump’s tweet amongst a certain crowd . . . are truth.”

Finally, the president-elect himself, who at a rally to celebrate his successful bribing of Carrier to keep some jobs in the United States, explained that he was impelled to act by a Carrier-employed supporter who had been naive enough to take Trump’s promises seriously.

Watching the evening news, Trump said, he saw the Carrier worker say “‘No, we’re not leaving, because Donald Trump promised us that we’re not leaving,’ and I never thought I made that promise. Not with Carrier.”

Then, Trump said, “they played my statement, and I said, ‘Carrier will never leave.’ But that was a euphemism. I was talking about Carrier like all other companies from here on in.”

This was a telling moment, and not just because Trump doesn’t quite understand what euphemism means. The episode simultaneously shows Trump, confronted with Trump on tape, willing to recognize reality and Trump telling us straightforwardly that his promises are not to be taken seriously. They are truthphemisms.

Of course, Trump is not the first truth-impaired president. Ronald Reagan famously insisted on repeating tall tales; he conflated Hollywood with reality. “If you tell the same story five times it’s true,” said White House press secretary Larry Speakes.

And Arendt reminded us a half- century ago about the inherent tensions between truth-telling and political power: “No one has ever doubted that truth and politics are on rather bad terms with each other, and no one, as far as I know, has ever counted truthfulness among the political virtues.”

But today we have the conjunction of a president unconstrained by facts with a media environment both siloed into partisan echo chambers and polluted by fake news. This development poses an urgent challenge — for journalism and for democracy.

The journalist’s challenge is not to tire in refuting the torrent of lies. The citizen’s challenge is to remain vigilant against the enticing lure of post-truth politics, to recall the admonition of our second president even as our 45th seeks to prove his wisdom an outmoded relic of a pre-post-truth era.

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