Mental illnesses are real. But the construct of “mental illness” isn’t. There is no such thing as an “illness” that is completely psychological in nature, ie. only “caused by thoughts and behaviours”.
What are called mental illnesses belongs into three broad categories instead:
Biological Illnesses
Many “mental” illnesses are genuine biological illnesses that have been shunned from fields such as neurology and stigmatised by calling them mental.
Ie. Schizophrenia (part genetic, several brain changes), Bipolar (genetic, HPA axis dysregulation + structural signs), Major depressive disorders etc. I’d like to remind that many genuine illnesses that dont even affect the brain were called mental illnesses before we fully figured the pathology out. From peptic ulcer to lupus.
difficult living conditions manifesting through changes in behaviour
ie. Some cases of anxiety disorder (maybe its normal to be anxious in the case you’re living, ie. stressful 9-5 with lots of responsibilities), reactive depression (it isn’t a mental illness to be depressed when your spouse dies, its completely normal)
Normal behaviours that society chooses to brand as deviant
ie. Gender dysphoria is not a mental illness, it is NORMAL, Same thing as homosexuality was called a mental illness in the past
My understanding of schizophrenia, as someone who has it, is that it’s a disorder affecting primarily the brain but we understand it very poorly. Even clinically, we’re more or less throwing darts at a board when it comes to treating the illness. What we have now works, more or less, but there are new theories emerging to support different potential treatment avenues. I’m really looking forward to seeing where that research goes. In any case, though, given that its primary effects are mental, I think it’s safe to call it a “mental illness.”