There Are Exceptionally Sharp Octogenarians. Biden Isn’t One.

Joe Biden’s problem isn’t his age. It’s his ability to function.

America has known a number of exceptional octogenarians who have demonstrated the cognitive and physical stamina to serve in demanding leadership roles. In 1787, at age 81, Benjamin Franklin, who a few years earlier had negotiated a highly advantageous treaty to end the Revolutionary War and had recently invented bifocals, played a pivotal role at the Constitutional Convention, persuading the delegates to allow citizens of the brand-new United States to vote without any property qualification. At the convention’s closing, Franklin wrote one of the finest speeches of the early American experiment, urging compromise and a unanimous vote to support the Constitution.

That September, the Philadelphia socialite Elizabeth Willing Powel asked Franklin, “What have we got, a republic or a monarchy?” In response, Franklin spontaneously delivered an immortal description of the United States: “A republic if you can keep it.” Until his death at age 84, Franklin continued to invent new gadgets and write lucidly, humorously, and perceptively, including a petition for Congress to end the slave trade and a satiric takedown of southerners’ justification for slavery, published just weeks before he died.

Franklin was an outlier. At an age when cognitive decline is common—when people tend to face challenges performing everyday activities such as managing money and organizing medications—he retained his capacity for mental focus, creativity, and sustained intellectual engagement. The same cannot be said of President Joe Biden. His disastrous debate performance last week indicated possible cognitive problems that interfere with his ability to function. Since then, his public speech has vacillated between energetic clarity and outright confusion. Some 80-year-olds are still sharp enough to be president. Biden has shown that he is not one of them.

Biden, following in Franklin’s footsteps, is a remarkably active octogenarian. At age 79, he negotiated the landmark Inflation Reduction Act. Now, at age 81, he is supporting U.S. allies through wars in Ukraine and Gaza. In the months leading up to the debate, he has flown around the country to campaign for reelection, and around the world to meet with other heads of state. But the debate featured telltale signs that his age is catching up with him.

As people age, they tend to retain what is called crystallized intelligence, the knowledge and skills that accumulates over a lifetime. Barring any brain injury or neurodegenerative disease (Alzheimer’s and its ilk), one’s vocabulary, general storehouse of facts, and recall of how to do things, such as knitting and skiing, remain robust—and may even improve—into old age. Conversely, even in the absence of disease, a different set of cognitive skills—fluid intelligence—tends to peak in a person’s middle years and then progressively decline with age.

[Read: The kind of smarts you don’t find in young people]

Fluid intelligence describes our ability to pay attention, exercise mental flexibility, and solve problems. People use fluid intelligence when faced with unfamiliar information that must be organized, and when they have to solve a new problem or navigate a familiar one in unusual circumstances. It’s what allows us to focus on important information or tasks while ignoring extraneous factors, and to hold one thing in mind while manipulating it, as we do when, say, calculating a tip.

In the debate, Biden displayed a striking deficit of fluid intelligence. He showed problems concentrating, difficulty with verbal fluency at the end of almost every response, an inability to spontaneously recall information, poor reasoning concerning issues that he was asked about, and a failure to respond to unexpected challenges by Donald Trump. Crucially, the debate was not unique. Biden has had good and bad days throughout his presidency, but diplomats, journalists, and even Democratic lawmakers have noticed and commented on his growing cognitive problems for many months.

Trump also displays many of these mental weaknesses—and others. His recall of names and events is poor, as is his capacity to concentrate, maintain his attention, and reason about new situations. Just consider his recent assertion that electric batteries could sink boats (and enable shark attacks), or that battery-powered airplanes would be grounded by the mere presence of clouds. But Trump’s displays of cognitive lapses have in many cases been eclipsed by his shameless, chronic lying. He appeared more energetic and lucid than Biden at the debate, but his answers were stuffed with factually false claims—thrice as many as Biden made.

[Read: Trump rants about sharks, and everyone just pretends it’s normal]

The different components of fluid intelligence begin to decline at different ages—mental-processing speed, for example, appears to peak in your 30s—and the exact timing and speed of that decay is highly variable from person to person. Some people’s mental slowdown becomes more pronounced in their 60s. For others, fluid intelligence remains strong well into their 80s. The rate of decline depends on many factors, including genes, physical activity, and education. Brain diseases such as Alzheimer’s and Lewy body dementia will accelerate the decline. Biden’s cognitive changes do not necessarily indicate dementia or neurological disease. (The White House has denied that the president has Alzheimer’s or any other form of dementia.) His performance is perfectly consistent with normal aging. And that is just as worrisome.

We need not go back 250 years to find examples of prominent older Americans with strong fluid intelligence. Warren Buffett is 93 and continues to be a successful investor, perceptive commentator, and masterful speaker at his shareholder conclaves. Nancy Pelosi is 84 and remarkably sharp. In her most recent term as speaker of the House, which ended when she was 82, she managed the passage of the American Rescue Plan and shepherded the bipartisan CHIPS and Science Act. She often went toe-to-toe with a younger President Trump and outshone him in wit. And the late Supreme Court Chief Justice William Rehnquist served capably in that role until his death at 80.

Early in his presidency, Biden asked perceptive questions, concentrated fully, and was engaged and lucid in his comments. I spoke with him in small groups on Zoom several times during the 2020 campaign, and was honored to serve on his COVID advisory board in 2020 and 2021. I did not notice any of the problems that have become so apparent in the past week. But now he has clearly deteriorated. His recent declaration that he will avoid events after 8 p.m. suggests that, somewhere inside, he recognizes the decline too.

[Adam Serwer: Biden must resign]

As a politician, Biden has been engaging in debates and Q&A sessions for some 50 years. His responses on expected topics such as abortion should come easily. Yet a slight change in setting—a silent stage with only Trump and CNN moderators for company—was enough to make his comments on the subject display a staggering failure of fluid intelligence: “Look, there are so many young women who have been, including a young woman who just was murdered, and he went to the funeral. And the idea that she was murdered by an immigrant coming in, to talk about that. But here’s the deal. There’s a lot of young women are being raped by their in-laws, by their, by their spouses. Brothers and sisters, by—it’s just ridiculous. And they can do nothing about it. And they try to arrest them and they cross state lines.” Such incoherence points to the kind of deterioration that was not known to have plagued Franklin or Rehnquist at Biden’s age.

In the days since the debate, people close to Biden have insisted that he is as sharp as ever, if not at every hour of every day. Regardless of the cause—normal aging, disease, or both—people with declining fluid intelligence can experience fluctuations in their day-to-day functioning. But an elderly person’s loved ones, and especially their family, tend not to recognize their deterioration until it is advanced. They see the person daily, so small changes often go unnoticed. They also accommodate their expectations to the decline. They tend to not be good diagnosticians unless asked very specific questions about daily habits and symptoms. But the rest of us, the American public, were shocked and shaken by what we saw, especially in contrast to Biden’s relatively strong State of the Union performance just a few months ago.

[Read: Joe Biden’s ‘cognitive fluctuations’]

The White House and the Biden campaign have suggested that the president’s problems at the debate stemmed from an exhausting travel schedule and a cold. Such explanations do not inspire confidence. Yes, it’s common for elderly people to bounce back more slowly from stressors. But even if jet lag and illness exacerbated normal cognitive limitations, said limitations remain, ready to surface again. And who knows when the next life-and-death decision will need to be made? Crises don’t wait patiently for presidents to be fully prepared. Someone whose cognitive competencies can be compromised as badly as Biden’s were by routine travel and a mere cold may be able to live a normal life, but they’d be hard-pressed to endure the rigors of negotiating with Congress or a foreign leader, much less making multiple rapid decisions when some future domestic or global disaster emerges.

In 1796, at the end of George Washington’s second term, he knew that the public would have elected him again and again, as many times as he wished. But he shocked the world by voluntarily relinquishing his executive authority. It made him a hero for all time.

Biden should take Washington’s example and withdraw from the 2024 race. In so doing, he can teach the world how to rise above politics, to sacrifice for the greater good. He, too, could become a hero. This election will determine the fate of the democracy in this nation. Many talented Democratic leaders from swing states could beat Trump. One of them should take the baton from Biden. That would truly solidify Biden’s legacy as a public servant and a successful president.