On Saturday, Nov. 4, 1994, the people of the United States were greeted with a startling letter.

“My Fellow Americans,” it began humbly. It did not waste any time for its purpose: “I have recently been told that I am one of the millions of Americans who will be afflicted with Alzheimer’s Disease.”

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It was Ronald Reagan, the conservative icon and president of the United States for eight years. Reagan wrote this letter. There is no doubt, since it was witnessed by Reagan’s then-chief of staff, Fred Ryan, Nancy Reagan, and the attending physician, Dr. Oliver Beahrs.

The Reagans set somewhat of a precedent in always telling the American people of their health in real time, from his status during the 1981 assassination attempt to Mrs. Reagan’s and his bouts with cancer.

Reagan’s telling the American people of his Alzheimer’s was in character for him and them. After all, they shared everything for 50 years and, beginning in 1994, they shared his slow descent into the abyss.

In some ways, it was a more trying time for Mrs. Reagan. Reagan left office almost six years prior to announcing his Alzheimer’s, but he did not think of himself in that letter. He was thinking of the American people in making his startling announcement.

Most recently, President Donald Trump invoked Reagan again, this time when Reagan’s name might have helped deflect critics claiming Trump could be the victim of early-stage Alzheimer’s. Reagan cited his ailments publicly so that the mention might help others. Trump referenced Reagan’s ailments to help himself.

Yet nearly 24 years after that tender and historic letter, and 13 after his death, few Americans have forgotten him. A recent poll of the American people showed an extremely high regard for his presidency, much higher than the presidents who followed him, including Trump.

Reagan’s approval rating at the end of his presidency was 63 percent, higher than all succeeding presidents, only marginally surpassed by Bill Clinton in 2001. Even as the American people know of and regard Reagan as beloved, there are still some, like Trump’s White House physician, Dr. Ronny Jackson, who, inadvertently or not, recently peddled the falsehood that Reagan suffered from Alzheimer’s during his second term.

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Look at this video of Reagan addressing students at the University of Virginia in 1988 — and just try to make the case that this is a man suffering from Alzheimer’s.

It would be nice to dismiss Dr. Jackson’s ill-informed and without-substance comments and move on, just as the White House would like to put the hearsay concerning Trump’s mental health behind it.

But he and Trump’s tweets have now given new life to an old and most-thought extinguished and ridiculous rumor. So once again, Reagan historians have to beat it down. Dr. Jackson’s uninformed comments made news and, even worse, they were uttered by a White House physician, giving the veneer of officialdom.

Jackson’s comments were clumsy and the irony is, he could have availed himself of his office’s files at Bethesda Naval Hospital on Reagan and seen for himself what Reagan’s doctors said about the Gipper.

The Trump White House could have easily cleaned up this mess, but so far … crickets. Some believe it serves their purpose to boost Trump by running down Reagan.

Predictably, some in the media — Gail Collins in The New York Times, for instance — have now reported it as fact, just a day later, pointing to Reagan as “being forgetful.” Here we go again.

It’s nothing more than that long list of conspiracies and legends and myths that cling to many presidents.

President Dwight Eisenhower was accused of having an affair with his World War II driver, Kay Summersby. Never. She said so in her own diaries. John Kennedy was killed in Dallas by the CIA, the Mafia, by two or three gunman, and not the crazed Lee Harvey Oswald — even as 100 percent of the evidence shows Oswald acted alone.

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Harry Truman supposedly (but never) said, “If you want a friend in Washington, buy a dog.” George Washington didn’t chop down a cherry tree and didn’t lie about it. FDR didn’t know about the attack on Pearl Harbor in advance.

“Reagan had Alzheimer’s during his presidency” is simply another to add to the pile of presidential mythology.

The American Psychiatric Association informally named a section of its Principles of Medical Ethics manual the “Goldwater Rule.” In 1964, a tabloid-like headline appeared in the magazine Fact against GOP presidential nominee Barry Goldwater.

The headline declared in loud, bold type that “1,189 Psychiatrists Say Goldwater is Psychologically Unfit To Be President!” Not one of the 1,189 psychiatrists had Goldwater as a patient, and it’s doubtful any even met him once. They placed fame over accuracy, and they did it for partisanship, medical professionalism be damned.

Dr. Jackson, and some in the media, have fallen into that same trap of playing Monday morning doctor to the deceased President Reagan.

The APA took issue, and in its manual, section 7, it has the “Goldwater Rule”: “On occasion, psychiatrists are asked for an opinion about an individual who is in the light of public attention or who has disclosed information about himself/herself through public media.

“In such circumstances, a psychiatrist may share with the public his or her expertise about psychiatric issues in general. However, it is unethical for a psychiatrist to offer a professional opinion unless he or she has conducted an examination and has been granted proper authorization for such a statement.”

It should be updated for Reagan. Call it the Reagan Rule: It’s unethical to diagnose the mental health of a president whose doctors diagnosed him as perfectly fit and has been dead for nearly 15 years.

Related: The Real Meaning of the Reagan Family Bible

Because if Reagan did — and we must emphasize he did not — have early stages of Alzheimer’s in the 1980s, then that would include as big a conspiracy and cover-up as JFK. Reagan had four physicians personally check on him, and every report came back stellar.

A man with Alzheimer’s does not write for his last journal entry as president these beautiful words: “Then home and the start of a new life.” He was 77 years old but looking forward to the next chapter in his life.

He knew where he was, what he accomplished, where he failed, where he could have done more. He had the faculties of an old, but wise, man, who knew that the presidency was an important part of his own and America’s life.

Call it the Reagan Rule: It’s unethical to diagnose the mental health of a president whose doctors diagnosed him as perfectly fit and who has been dead for nearly 15 years.

But no, all that was overshadowed by his nonexistent disease.

To carelessly speculate about Reagan as Trump’s doctor has now done results in the very same effect as the thinly veiled attempts to delegitimize all his work, his administration, his life. He did something conservative? Oh, it was the “Alzheimer’s.”

This is the same man who successfully crafted the policies that drove the GDP to increase from negative 0.3 in the last year of Jimmy Carter’s presidency to 4.1 percent in 1988. Inflation, which was 13.5 percent when Reagan was inaugurated, dropped dramatically. The Soviet Union was on its knees, only to fall with the wall (a very real wall) a couple of years later. Relations restored with world leaders and the blight of communism was put in sight.

There is an old phrase that says, “A lie can make it around the world while the truth is just getting out of bed.” Let’s instead put the lie to bed, now and forever: Ronald Reagan was many things — almost all very good — but he did not have Alzheimer’s during his presidency.

Craig Shirley is a New York Times best-selling author and presidential historian. He has written four books on President Ronald Reagan, along with his latest book, “Citizen Newt: The Making of a Reagan Conservative,” about the early career of former Speaker of the House Newt Gingrich. He lectures frequently at the Reagan Library and is the Visiting Reagan Scholar at Eureka College in Illinois, the 40th president’s alma mater. He also wrote the critically acclaimed “December 1941.”

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