ANY political system that puts great power in the hands of a single person must also reckon with the problem that creates. It is sensible to make provision to remove the king or president from office if he becomes incapacitated. Yet the existence of such a provision also risks inviting a coup. The framers of the constitution were acutely aware of this, and decided to fudge it. Meanwhile, in Britain, George III’s bouts of mania invited questions about who is really in charge when the monarch is on the throne but out of his mind. Speculation about President Donald Trump’s mental state, ever-present since before his election, increased when he tweeted that his nuclear button was “much bigger” than Kim Jong-Un’s. Being Mr Trump, he followed up with his own self-analysis, declaring that he is a “very stable genius”.

Most psychiatrists are wary about pronouncing on the mental state of people they have not examined, but that has not stopped a few from having a go at Mr Trump. Bandy Lee, a psychiatrist at Yale and editor of a book called “The Dangerous Case of Donald Trump”, briefed members of Congress before Christmas. More than 50 Democrats have signed up to a bill to compel the 45th president to submit to an examination of his fitness for office.

Speculation about what is on the president’s mind begins with assertions that he might be in the early stages of dementia. Those who argue this begin with the observation that the president’s father had Alzheimer’s disease. That is not, by itself, strong evidence. There is a version of Alzheimer’s that is almost guaranteed to be passed on from parent to child, but it tends to show up in people in their 40s and 50s. The other strains are less strongly heritable, meaning family history is a useful part of a diagnosis, but not more than that.

Then there are observations based on the president’s use of language. When transcribed, Mr Trump’s extempore speeches are unusually jumbled. But that might always have been the case: it is hard to be certain because his voluminous published writings were ghostwritten by someone else. Becoming less lucid when speaking is a fairly normal part of ageing. Researchers report that tip-of-the-tongue moments, when a speaker struggles to find the word he is looking for, become more frequent with each passing decade.

In theory it would be possible to take the president’s speech patterns, run them through software and then compare the change over time with what might be expected as part of normal ageing. (Doctors have been through George III’s correspondence and found that he had periods of extreme logorrhoea when he was unwell, writing sentences consisting of 400 words with only eight verbs.) There are two problems with this. First, there is no established baseline for what normal ageing looks like. Second, Mr Trump’s changes of career, from property developer to TV star to presidential candidate, would naturally lead to a change in how he uses words. A proper test for early-stage dementia would require several different types of brain scan, reasoning, memory and genetic tests. In the view of a leading researcher who runs an institute dedicated to early diagnosis and treatment of dementia, it would be “highly irresponsible” to diagnose anyone on language use alone.

If the evidence for dementia is thin, what about that other frequent diagnosis—that the president has narcissistic personality disorder (NPD)? The checklist for this syndrome in the fourth edition of the Diagnostic and Statistical Manual (DSM) includes: “has a grandiose sense of self-importance”, “is preoccupied by fantasies of unlimited success”, “believes that he or she is special and unique”, “requires excessive admiration” and has “unreasonable expectations of especially favourable treatment or automatic compliance with his or her expectations”. That might sound familiar. “I alone can fix it,” Mr Trump declared at his nominating convention in 2016. John Oldham of Baylor College of Medicine, who presided over the compilation of the chapters in DSM-5 on personality disorders, says that the president’s behaviour (especially his exaggerated need for admiration and praise) “reflects things we see in people with NPD”.

Yet Dr Oldham also cautions that a lot of successful people have a touch of narcissism. What he calls “healthy narcissism” tips over into disorder territory when it impedes a person’s ability to form normal bonds with other people. By way of example, Dr Oldham mentions a patient whose idea of meaningful interaction with family members was to give extravagant gifts. This man sought help after his son attempted to commit suicide and his wife threatened to leave him. Unlike asthma, people who have NPD do not think they have it, and there are no drugs recommended for treatment. Unlike dementia, it need not impair memory or basic reasoning.

Whatever Mr Trump is thinking about, he is a long way from 25th-Amendment territory. This is the provision of the constitution added in 1967, after Woodrow Wilson’s strokes, Dwight Eisenhower’s heart attacks and John F. Kennedy’s assassination. Clause three of the amendment covers what happens when the president knows he will be unconscious; it has been used three times, once when Ronald Reagan underwent medical procedures and twice when George W. Bush did so.

Clause four is designed to deal with cases where the president is so out-of-it that he is unable to hand over authority. Again, clause four’s authors were not specific about what might legitimately trigger it, and they worried about coups. But clause four seems designed for a situation where the president is either rendered unconscious, or is wandering around the White House unable to recognise his own reflection. Even then, to make the 25th Amendment stick still requires super-majorities in both houses of Congress.

The rush to diagnose the president might be taken as evidence of Trump-derangement syndrome: the president’s critics find him so maddening that he drives them to despair. There is also a mild irony here, of a kind that partisan thinking often throws up. Left-leaning advocates for more humane treatment of the mentally ill tend to argue that mental illness is a normal part of human experience, and that a touch of one disorder or other ought not to disqualify someone from doing an important job (depressed pilots and epileptic lorry-drivers aside). Yet left-leaning critics of the president are, in this case, arguing that he should be declared incapable.

There are plenty of reasons to think that Mr Trump is ill-suited to the presidency. These have become more apparent over the past year, but all were there before his election. Madness has little to do with it.