I don't think much depends here on whether psychology or psychiatry is a science. What matters is whether they are doing good or not for patients. Supposing we think they are, a commonsense criterion seems to be that even if patients do not accept the diagnosis or the treatment, things change if they are contemplating harm to themselves or others, as the jargon has it. The jargon persists and is more than jargon because it has a real role; it is a criterion. A second criterion or perhaps test might be this. Can the patients get through the day or week to their own satisfaction, or look after themselves OK? If I have a friend who never gets up, doesn't bathe, won't look me in the eye, can't work and doesn't eat, and has gone down to eighty pounds, then I feel justified in intervening, because of this second criterion, over the stated wishes of the patient. Something is wrong, and I have an obligation, or some responsibility anyway, to help, especially if I am a psychiatrist. Most people in trouble do want help, I think, even if we are too dim to find the way. "Normative" ethics means ethics which is not-metaethical. I think you may be misusing the word; what you may mean is a highly directive ethics that conflicts with the patient's own ethics. The patient too has a normative ethics - his own. And what if psychiatry is misguided or "pseudo-medical" and does harm? If that is true, then we should avoid it, of course. But is it true? It seems to me it is a bit improbable that the profession as a whole is phony. It is not monolithic enough for that, for one thing. One can always switch psychiatrists.
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If we as professional and amateur philosophers are to accept psychology as a science (see question 5362 answered by Charles Taliaferro), then what does that make psychiatry? Even if a person fits all the criteria for having a mental disorder according to principles of psychology yet he refuses treatment, why should medical or pseudo-medical psychiatrists intervene so long as that individual decides to refuse to concur with the diagnoses or more importantly refuses to to acknowledge the treatment while accepting the diagnoses? Isn't psychiatry then just a mostly futile exercise in normative ethics to the whim of the patient?