I may be wrong, but I have a sense that your key interest is the extent to which matters of mental health are grounded in nature or in a reality that is independent of changing or contingent matters, right? I am checking in to make sure I get the question, for there is a sense in which if you are asking about whether the term 'health' is defined by society, the reply should probably be that 'health' like all terms (from 'dog' to 'mountain') in all languages is indeed defined by social conventions. But it is a further matter whether 'health' or 'mental health' refers to something that is not only a matter of social convention. If I am right about your concern, then I think we have reason to believe that there are some norms that define 'mental health' and 'mental illness' that appear to range over cultures (or that cultures hold in common) and are pretty basic, e.g. it is not mentally healthy for parents to torture their children or for pilots to deliberately crash an airplane full of passengers into the Alps killing all aboard or for persons who are otherwise healthy to wash their hands every two minutes due to no apparent reason, and so on. But apart from some basic, widespread (perhaps even "common sense") boundaries, there can be significant reasonable disagreements. To take an extreme case, Richard Dawkins contends that those who believe in God are subject to what he at least used to call "the God delusions" --as he is not a psychologist or psychotherapist, consider Sigmund Freud who contended that religious believers were subject to a pathology, namely the Oedipal complex. Many of us (including me) thinks this charge is groundless and reflects either mistaken philosophical assumptions or (using stronger language) unwarranted prejudice. So, overall, I think that while there are some basic boundaries between mental health and illness, there is a great deal to debate. On the job of psychotherapists, insofar as she or he is committed to the health of her clients (or fellow citizens) I suggest the therapist would have reason to discern when some norm is merely socially contingent versus one that is deep and not subject to social variation. Sexual orientation may be a good case in point. If a therapist is working with a client who has homosexual tendencies and yet is working in a society that deems homosexuality a mental disorder, then the therapist may indeed have a job in proposing that the current "beliefs of the society in which s/he operates" are ungrounded or contingent.
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to what extent is the definition of mental "health" conditioned by society and social mores? To what extent is the job of a psychotherapist grounded in and/or free from the beliefs of the society in which s/he operates?