Why the distinction between our brains and every other part of our body? Particularly related to illness when the question is often asked "is _______ physical or is it “mental?”
Functioning well or not functioning well are not all my components physical?
Because every other parts of your body primarily operate in a manner dealing with the laws of Newtonian physics (i.e. “moving stuff”). While your brain does as well, most of what it does is the largely invisible processing of data.
There is a physical brain, and it can be physically injured.
There is also a mind, which is intangible, and it can also be injured. This is “mental.”
What is the confusion?
Your brain is indeed physical, and subject to the same functions and dysfunctions as the rest of your body. But then there’s your MIND, which is probably what you’re referring to as “mental.”
Bah! The mind is a fiction. Like the “spirit.”
Psychiatrists deal with hardware issues.
Psychologists deal with software issues.
Brain and brain! What is brain?
Go to your room!! 
You see? You see? Your stupid minds! Stupid! Stupid!
Actually, there’s a movement to refer to to things not as “mental” but rather as “behavioral.” In other words, a disease can have physical symptoms, or behavioral symptoms. And it might be a coin toss. For example, in a ten-year-old boy, wakeful incontinence is probably going to get labeled a behavioral symptom, while in a 50-year-old women, it will be labeled physical, because of assumptions. It is assumed that bladder infections are extremely rare in boys who are not sexually active, but even a boy who is not being molested could get one if, for example, he has IBS and is having diarrhea in the bathtub (happened to a kid I know). Also, in a 50-year-old woman, who is many years post-menopausal, and doesn’t have a UTI, the incontinence could be the first symptom of early onset Alzheimers.
So even those labels can be wrong, but at least for the time being, they are less loaded, and lead to further investigation, not immediate, sometimes inappropriate treatment.
Also, sometimes the same disease can have both behavioral and physical symptoms, and doctors are now recording behavioral symptoms, because the next time the person gets the disease, the behavioral symptoms might show up first. For example, in some children, strep causes either ADHD-like symptoms, or OCD-like symptoms, and those show up before a fever or sore throat (even, in rare cases, instead of). In those children, it is important that this be documented, because if a child who does not normally exhibit ADHD behaviors suddenly starts exhibiting them, they may need an antibiotic, not Ritalin. More importantly, they need the antibiotic before the strep does systemic damage, just like a kid whose first warning sign is the more typical sore throat.
So, well, I need a concluding sentence. I don’t know what to say, except that people in the field have actually been using “behavioral” for a long time. Like, since the 20th century. ADHD, schizophrenia, and such, have been “behavioral” disorders for a long time, although there is still a distinction between “psychiatric” (brain chemistry) and “neurological” (brain structure) disorders.
True. If somebody has a stroke, we don’t call that a mental illness.
A host of physical problems are rooted in psychological causes. The digestive tract is probably the most commonly affected. Would that be an example of behavioral?
Autism. It’s a neurological, not psychiatric condition, whose symptoms are about 95% behavioral. Down Syndrome is also a neurological condition that is mostly behavioral, but because it has the physical symptom of a unique and defining countenance, it is considered very different from autism, although it is actually very similar.
And yes, before you ask, there are some autistic syndromes that have physical symptoms. The autistic population is about 25% more likely than the average population to have Irritable Bowel Syndrome, and they often have some physical delays, although they are not as significant as the developmental delays: but often in isolation autistic people have delays that would merit a diagnosis of dyspraxia in a non-autistic person. Dyspraxia is not nearly as serious as say, CP, but it often merits similar interventions, and requires a person two be two standards deviations below the norm in a physical target, like being able to run a certain distance in a certain time, or be able to stand on one foot.
My son has dyspraxia. He had hypoxia at birth, and he has had some interventions, and is on target now, but he needed, for example, to be taught to put on pants because he couldn’t stand on one foot. He’s 10, and can do it now, but he couldn’t until he was 6 or 7. He couldn’t ride a bike until I got a tandem to teach him.
If he were autistic, those things would be considered part of his autism, and he could still get therapy, but he wouldn’t have a separate diagnosis of dyspraxia.