Legal tests of capacity
The legal test which we now have was formulated in law in 1994 in the case of Re C [9]. Mr C had bad gangrene in his leg, and the doctor said he had an 85 per cent chance of death if his leg wasn't amputated. And Mr C said, "You're not chopping my leg off!" So you might think, "Well, what's the problem? We have this right to autonomy; it should be upheld."
The thing about Mr C was that he had paranoid schizophrenia and was a detained mental patient in a maximum security hospital. So, to paraphrase, the doctors said, "You're mad, so we can chop your leg off if we think it will help."