...It only matters that Trump is unfit for office. The "Is Trump nuts?" argument, as Josh Marshall correctly points out, is ultimately meaningless and a distraction from the truth.
One of the diagnoses you often hear tossed around, rightly or wrongly, is Narcissistic Personality Disorder (NPD), a Class B personality disorder. I think most psychologists and psychiatrists would tell you, privately if not publicly, that a number of Trump’s behaviors could (I stress, “could”) be explained by NPD. But that doesn’t tell us that much. Lots of symptoms and behaviors can be explained by many different diseases and disorders. Or no disorder or problem at all. That’s why you need a proper examination. (This applies of course to both somatic and mental illnesses.) Some shrinks may say they’ve seen enough to know; others would say, no, never without a full examination. Again, for our purposes, it doesn’t matter. If the entire psychiatric profession got together and examined Trump and pronounced him entirely free of any mental illness, his behavior wouldn’t be any less whacked or dangerous in a President.
That brings us back to the point. It’s really only the behavior that matters to us as citizens. A diagnosis would only be helpful to learn about behavior we don’t know about or predict future endangering behavior. Since we know about the behavior we’re talking about, none of that matters or applies. In common sense, every day rather than clinical language Trump is clearly unstable, erratic, impulsive. In a word, he’s nuts and not well. As citizens, we are entirely able and entitled to make these determinations. They are ordinary English language descriptors that the psychiatric profession doesn’t control and shouldn’t want to control. The entire debate over whether Trump is “mentally ill” is simply a diversion, premised on the idea that we need either permission or dictation to say he is not able to safely or competently fulfill the job of President. We don’t. The observed behavior is really all that is necessary and all that matters. It’s very clear.
In other words, the reason why Trump is unfit for office as demonstrated by his behavior in office is irrelevant. The key here is that Trump is unfit for office, and should be removed from it.
We have three mechanisms to remove Trump: the 2020 election, impeachment on high crimes and misdemeanors and then conviction/removal by the Congress, and removal under the 25th Amendment. The latter two are both controlled by Trump's party, and neither one will be used unless an overwhelming majority of the American people start calling for that to happen. We're not at that point yet.
The real problem is that Trump's unfitness for office is obvious to the party that nominated him, and yet they will do nothing. That is the failure of the American political system right now, and one that will cost us for decades, if not generations.