Biden’s memory isn’t worse than Trump’s (or Reagan’s), but it seems that way. Here’s why.

“The more you learn, the more you know,” the superstar physicist Stephen Hawking once observed. “The more you know, the more you forget.”

I’ve been thinking about Hawking’s remark during the run-up to President Joe Biden’s State of the Union address on Thursday. A source close to Biden said the president will try to neutralize the “negative aftershocks” of last month’s report by special counsel Robert Hur, who declined to charge Biden with mishandling public documents but called him a “well-meaning, elderly man with a poor memory.”

Republicans seized gleefully on Hur’s report, which seemed to confirm a well-worn talking point on right-wing media: At 81, Biden is just too old and feeble for the job. Several prominent GOP leaders called for Biden’s removal under the 25th Amendment, which provides a procedure for replacing presidents who become incapacitated.

In reply, Democrats noted that Donald Trump — the putative GOP presidential nominee — has also seemed baffled and confused. And during Trump’s presidency, his critics likewise demanded that he be removed under the 25th Amendment — especially after he encouraged the assault on the U.S. Capitol in the waning days of his term.

But here’s the big difference: Biden knows more than Trump, so he also has more to forget. And that’s also why voters say he’s less able, as a poll released last weekend shows. His gaffes and memory blunders suggest decline, whereas Trump’s mistakes can be chalked up to his ignorance.

Call it the obverse of the Hawking principle: The less you know, the less you have to forget. And that makes the public more likely to give you a free pass as you age.

Just ask Ronald Reagan. At 69, Reagan was the oldest American president to enter the White House until Trump (70) and Biden (78). Reagan routinely made mental lapses (and was diagnosed with Alzheimer’s disease after he left office), but Americans didn’t much care. They had come to expect it from a man who was fundamentally dreamy, detached, and incurious. As one of Reagan’s own children admitted, “He makes things up and believes them”; he would “build these little worlds and live in them,” a senior adviser confirmed.

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Reagan had a much sunnier disposition than Trump, of course, who is a tempest of rancor and resentment. But both of them have lived in fantasy worlds that make it difficult — if not impossible — to perceive any kind of debility. If someone is already checked out, how can you check if they’ve gotten worse?

Nearing the end of his first term as governor of California, Reagan was unable to tell a reporter a single legislative achievement of his administration. But the voters returned him to office anyway. “He believes he’s above it all,” an astonished campaign aide said. “That’s why they believe it. I can’t believe it. But they do.”

In 1981, at a White House meeting with big-city mayors, Reagan greeted Samuel Pierce — his secretary of housing and urban development — as “Mr. Mayor.” It could have been an embarrassing moment, especially because Pierce was his only Black cabinet member. But not for Reagan, who made a joke out of it. And everyone in the room laughed, including Pierce.

During his 1984 reelection campaign, newspapers started to suggest that Reagan was “showing his age,” as a Wall Street Journal headline noted. Television reports picked up on the cue, running a clip of Reagan dozing off in front of Pope John Paul II.

Once again, Reagan defused the matter with a joke. “I will not make age an issue in this campaign,” he quipped during a debate with Democratic challenger Walter Mondale. “I am not going to exploit, for political purposes, my opponent’s youth and inexperience.” As Mondale recalled in a 1990 interview, that was the moment he knew he had lost. But he laughed anyway, along with everyone else.

In his second term, Reagan’s lapses became even more glaring. Asked by the Icelandic prime minister what he hoped to accomplish at his now-famous 1986 summit in Reykjavik with Soviet leader Mikhail Gorbachev — at a time when U.S.-Soviet relations were strained and the world was worried about nuclear war — Reagan replied that Iceland’s airline would be granted landing rights in Boston. Later that year, at a ceremony to celebrate his tax reform bill, he signed it by writing “Reagan” — not Ronald — first.

Again, no matter. As an assistant to French leader Francois Mitterrand remarked, following a meeting with Reagan, cluelessness was actually Reagan’s prime asset. “Formidable will, based on mediocre understanding of the facts,” the aide observed. “As often in politics, ignorance sustains strength.”

“As often in politics, ignorance sustains strength.”

Ignorance also insulates politicians from charges of decline. Borrowing from Reagan, Biden has tried to make jokes about his advanced age. But the voters aren’t buying it, because — put simply — they expect more of Biden. Unlike Reagan — or Trump — Biden has spent his life teaching himself about the world. It would be tragic if we punished him for forgetting what Trump never knew.

Jonathan Zimmerman teaches education and history at the University of Pennsylvania. He is the author of “Free Speech and Why You Should Give a Damn,” with illustrations by cartoonist Signe Wilkinson.