One major tradition in psychology says that mental illness doesn't exist because the mind is not an organ and so can't be diseased. But the brain sure can.
Here is where I learned something that I can't believe I hadn't come across before.
The background: There exists in rare cases a malady called "temporal lobe focal epilepsy." (Picture the temporal lobe by imagining that the brain, viewed from the side, looks like a right-handed boxing glove. The "thumb" is the temporal lobe. Focal simply means that the problem is sharply limited to one tiny area of the lobe. The term epilepsy is used because the disease causes that area to give off brain waves similar to those that cause epileptic seizers - but these waves have a far different result.)
The problem is the growth of a tiny calcified mass in a certain spot. It can make a person's personality change for as long as a full day from a normal person into an angry but seemingly otherwise highly organized criminal. They might lash out at others, smash property, or even take off on a senseless killing spree. When the episode passes, they have absolutely no memory of what they have done. And removal of the growth removes the disease forever.
Here is what blew me away. If you've been around long enough, you vividly remember news reports of the young man who grabbed a rifle, climbed atop a tower overlooking the center of the University of Texas, and shot his fellow students at will. Perhaps that should be, at "will," because an autopsy found that he had been a victim of temporal lobe focal epilepsy.
I don't know how it is that I never bumped into this information. It certainly has been around for a long time, and is used by those who argue that perhaps all "mental" illness is caused by a potentially treatable brain dysfunction. I'd never go so far as to suggest that all criminals are victims of a brain disorder and so should not be punished. That's ridiculous and would turn our justice system on its head. But for seemingly inexplicable crimes, maybe it's worth more consideration by the courts.