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
But if we’re talking about a child for instance who is suffering from mental illnesses brought on by repeated or extensive childhood trauma?
That might be most analogous to getting an infection after your legs are broken maybe? I think I’d consider it an illness, even if it’s a purely cognitive response to extreme trauma in ones formative years.