“Do you think it’s possible for a psychologist or psychiatrist to diagnose Trump with a mental illness just from observing him?”
This seems like the wrong question. A lot of psychology is done via observation, but psychological observation is not the same as the partial look into Trump’s mind that we get from his public appearances. A professional could probably discover what, if anything, to diagnose Trump with if they were given the opportunity to observe him to the level that scientific rigor requires. But if the question is closer to “do you think it’s possible for a psychologist or psychiatrist to diagnose Trump with a mental illness just from watching the portions of his public appearances that are covered on television?” then I think the answer is “no, and it would be unprofessional to do so.”
“That argument [that they have to have him in person]) has always struck me as nuts. There is no diagnostic blood test or brain scan for narcissistic personality disorder; there’s just a list of observable traits.”
This does not strike me as a good rejoinder to the argument being criticized for precisely the reasons I mentioned above. That narcissistic personality disorder is diagnosed by observation rather than through mechanical or chemical means doesn’t entail that a member of the general public—that is, someone who has not observed Trump extensively and in person—is capable of reliably diagnosing him based on what they’ve seen.
In any case, what would having a specific diagnosis change? Mental illness and incompetence are not the same thing, nor do they necessarily go together. Abraham Lincoln probably suffered from clinical depression, but his handling of the presidency can be evaluated without that knowledge. Similarly, I don’t need a diagnosis to determine which of Trump’s policies I agree or disagree with. I suppose it might be relevant to how we think of him as a person (as opposed to how we think of him as a politician), but I don’t get the sense that anyone eager for a diagnosis is actually concerned about Trump’s personal wellbeing.