At least(!!), Victor has noted the social nature of the definition of the ill. So thank you victor for advancing the thread.
Yes, most mental illness is a medical fiction.
Yes, it's a medical fiction just as we have legal fictions. For instance, the Corporation is legally a fictional person with all the rights and responsibilities of a person.
And the medical fiction of mental illness was created with the best will in the world. It was created to get the 'mentally ill' out of prison and into the medical asylums.
In other words, the medical fiction of mental illness was an improvement over the criminal model.
But we lived in a literate and so literal society and so the fiction of mental illness was taken literally.
And naturally as 'mental illness' now literally existed, it must have a physical basis. And of course you find what you look for.
Unfortunately most illnesses are caused by a lesion or an infectious agent. And so far we haven't been able to find lesions or infectious agents that cause most 'mental illnesses'.
But interestingly we are moving from a literate and so literal society based on books to an electronic society based on the telephone, television and the internet. And this is changing our view of everything, including mental illness.
So the history of mental illness starts with the idea of being 'touched' by a god or a demon, and moves onto the criminal model where we lock them up, to the medical model where we medicate them. And mental illness it now moving onto a new model which we are creating, in part, in our discussion.
So 'mental illness' has moved from being 'touched' to being 'bad' to being 'sick' to something we are now discovering.