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