It is hardly surprising when President Trump says something that turns out not to be true.

What has not always been clear, however, is what drives him to lie so constantly. Is it because he’s ignorant? Or because he is deluded? Or simply that he is malevolent? The possibilities are not necessarily exclusive of one another.

Now, thanks to The Post’s Bob Woodward, we have learned the answer with regard to what history is likely to rank as perhaps the most consequential of all the falsehoods that Trump has uttered.

In his soon-to-be-released book, “Rage,” Woodward reveals that Trump intentionally played down the threat from covid-19, which has killed at least 186,000 Americans since February and dealt a gut punch to the economy.

At a time when the president was assuring the country that this virus was no more serious than ordinary flu, and that his administration had the situation under control, he was telling Woodward a far different story.

On Feb. 7, the president called Woodward and told him that the coronavirus was “deadly stuff.”

“You just breathe the air and that’s how it’s passed,” Trump said in a Feb. 7 call. “And so that’s a very tricky one. That’s a very delicate one. It’s also more deadly than even your strenuous flu.”

In another interview, on March 19, Trump admitted to Woodward that he deliberately lied about the danger to the public.

“I think, Bob, really, to be honest with you, I wanted to always play it down. I still like playing it down, because I don’t want to create a panic,” the president said.

These are not a leak from anonymous sources. These are the president’s own words. You can hear them on Woodward’s tape.

Trump had been apprised of the truth as far back as Jan. 28, when he was briefed during a top-secret session in the Oval Office. At that point, the outbreak had not spread widely beyond China.

“This will be the biggest national security threat you face in your presidency,” national security adviser Robert C. O’Brien told Trump, according to Woodward’s reporting. “This is going to be the roughest thing you face.”

Matthew Pottinger, the deputy national security adviser, had been speaking to contacts in China and warned the president that he could be facing a health emergency as bad as the flu pandemic of 1918, which killed an estimated 50 million people worldwide.

But Trump continued to sell a far different line in public. On Feb. 10 — three days after he had told Woodward that covid-19 was “deadly stuff” — he predicted the virus would disappear by April: “When it gets a little warmer, it miraculously goes away.”

And on Feb. 26, shortly after the first cases were reported in this country, he blithely assured us: “We’re going to be pretty soon at only five people. And we could be at just one or two people over the next short period of time. So we’ve had very good luck.”

The consequences of Trump’s refusal to be straight with the country, we now know, have been tragic.

Rather than taking cautionary measures that could have spared immeasurable suffering, such as assuring that adequate testing was available, the president denied what he knew was true. Or, as he so memorably put it at a news conference in March: “I don’t take responsibility at all.”

In that case, however, Trump was telling us something fundamental about himself. Those six words are both the most fitting epitaph for his presidency, and the central reason that Americans should put an end to it when they go to the polls in November.

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