We are now back on to the feverish debate about whether or not Donald Trump is mentally ill or suffering from the onset of dementia. The most important thing to know about this debate is that it simply doesn’t matter. Diagnoses are something for trained professionals and even they are challenged to make them without a proper in-person examination. But again, it doesn’t matter.

For public purposes, clinical diagnoses are only relevant as predictors of behavior. If the President has a cognitive deficiency or mental illness that might cause him to act in unpredictable or dangerous ways or simply be unable to do the job, we need to know. But my God, we do know! We see him acting in these ways every day – and not just in multiple news reports from an abundance of different news organizations. We see it with our own eyes: in his public actions, his public statements, his tweets. All the diagnosis of a mental illness could tell us is that Trump might be prone to act in ways that we literally see him acting in every day: impulsive, erratic, driven by petty aggressions and paranoia, showing poor impulsive control, an inability to moderate self-destructive behavior. He is frequently either frighteningly out of touch with reality or sufficiently pathological in his lying that it is impossible to tell. Both are very bad.

It is now widely believed that President Reagan showed early signs of his later Alzheimers diagnosis during the latter part of his presidency. How much or whether it affected his ability to carry out his duties is less clear. But that is a very different case. The kinds of subtle lapses in memory or cognition that might hint at such an affliction would be very difficult for the public to be aware of, especially if his staff is making efforts to conceal them. We don’t have that problem here.

It is also important to understand the nature of mental health diagnoses. Neurological disorders and lapses of cognitive function are different and something I know less about. But a key element of most DSM-V diagnoses is the degree to which the behavior or affect impairs normal life functions. Can you hold down a job? Can you carry on healthy or functional relationships with other people? Can you sleep and take basic care of your health? This fact is important for a number of reasons. But it highlights an important fact: the nature of clinical diagnoses of mental illness are heavily informed by the need for and potential efficacy of treatment. The framework of these clinical definitions simply don’t line up well with the issues that are important to ask about public officials, which are more or less entirely about behavior.

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.