When US President Donald Trump bizarrely declared himself a "very stable genius" on Twitter last weekend, he opened a door for journalists who cover him.

Whether they should rush through it, arms waving and warning about the dangers of a mad ruler, remains a question. There's a fine line between taking up - in reporting and commentary - Mr Trump's fitness for office and outright speculating that he is mentally ill.

New Yorker editor David Remnick walked that tightrope in a tough-minded commentary piece last week in which he compared Mr Trump to the Roman emperor Nero, whom he described as "unhinged". He referred to Mr Trump as "chaotic, corrupt, incurious, infantile, grandiose". Vox editor at large Ezra Klein was more blunt: "The President of the United States is not well."

MSNBC host Joe Scarborough made headlines recently when he said he has tried to report about Mr Trump's possible dementia in columns for The Washington Post but wasn't allowed to do so.

"I've written twice in my column a quote about one of the people closest to Donald Trump during the campaign saying he's got early-stage dementia," Mr Scarborough said on the air. "He repeats the same stories over and over again. His father had it. And it's getting worse, and not a single person who works for him doesn't know he has early signs of dementia.

"But twice The Washington Post would not let me put that in my column. Which again, I salute them for having a high bar, but we're at this moment."

I asked Post editorial page editor Fred Hiatt about this. He told me he was uncomfortable with Mr Scarborough's reporting that suggested a specific medical diagnosis, especially since it would have been attributed to an unnamed source who was not a medical professional.

Mr Hiatt said the information was, in his view, not necessary to the larger concern about Mr Trump's fitness for office that Mr Scarborough was addressing. This is, no doubt, an issue that newsrooms everywhere are grappling with.

"It's perilous to go too far on this subject," Ms Carrie Budoff Brown, editor of Politico, said. "It's delicate, it's sensitive - and we're not doctors." She acknowledged, though, that readers "want this answered: 'Is he this or that?' ".

I heard similar reflections from both New York Times executive editor Dean Baquet and Washington Post executive editor Martin Baron. "I've been very wary," Mr Baron said, particularly of reporting that gives credence to those who offer diagnoses without examining or even having personal exposure to the President.

He observed that psychiatrist Allen Frances, who helped write the manual for diagnosing mental illness, has publicly stated that he doesn't think Mr Trump meets the criteria, writing in September that "the three most frequent armchair diagnoses made for Trump - narcissistic personality disorder, delusional disorder and dementia - are all badly misinformed".

"It's certainly fair to report on the overall subject," Mr Baron said, including what people who have observed the President directly are saying. And he agreed the calculus has changed somewhat now: "Trump himself has put it out there for discussion." (Mr Trump did so after Mr Michael Wolff's book Fire And Fury reported that the 25th Amendment - which allows the removal of a president for unfitness - is a regular topic in the White House because so many of those around Mr Trump believe he is mentally unstable.)

Mr Baquet said his guiding principle at NYTimes has been to stick with "reporting, not speculation", noting that the American Psychiatric Association's "Goldwater Rule" prohibits its members from diagnosing the mental state of public officials they have not examined personally.

The guideline itself, though, is under attack from those in the profession who say Mr Trump's instability poses a huge threat, making such diagnoses necessary for the good of the world.

"Of course, the Goldwater Rule is not our rule," Mr Baquet observed, meaning it isn't meant to apply to journalists, but its existence suggests caution nonetheless.

Not all in the field observe it: In 2015, clinical psychologist Ben Michaelis was quoted in Vanity Fair on Mr Trump: "Textbook narcissistic personality disorder."

Is Mr Trump a very stable genius or the modern-day Nero? The question is now fully out in the open, thanks to the President's own impetuosity. But news organisations still need to stick to show-don't-tell reporting focused on direct observation of what Mr Trump says and does, and conversations with people who interact with him regularly.

Shattering norms is what Mr Trump - and covering Mr Trump - has been all about. As journalists cautiously approach the line of what's previously been acceptable, it's often Mr Trump himself who pushes them over. But it's good to remember that when norms are shattered, they stay that way, like so many Pandora's boxes with their lids wide open for all time. And that it's possible for a president to be unfit for office without being mentally ill.

WASHINGTON POST