I think this is the perfect thread to pose this question: what is the subject matter of psychiatry? For example, cardiology is the study of the heart and the diseases that can affect it's proper functioning. Neurology is the study of the brain and central nervous system and its associated diseases. What does psychiatry study? You ask even mental health professionals and even academics in the field and you get a surprising variation in responses. Some say it's the study of human behavior, which is fine in and of itself, but how does that relate to the idea of the body as a machine? They use analogies like "dysfunctional behavior" which is sort of a misnomer, since all behavior has a function (what they really mean is "disapproved" or "maladaptive"). Some say that it's the study of the mind. But is mind even a scientific concept, or is it a philosophical idea? Others say that it's the study of the brain. Okay, but how does that differentiate psychiatry from neurology? They might then reply that neurology studies brain diseases that cause physical problems, like mobility issues, numbness and tingling and aches and pains, whereas psychiatry studies brain diseases that cause behavioral issues, like schizophrenia or bipolar. But don't all brain diseases affect behavior? And does it seem likely that a person could have a brain disease that affects only behavior, and does not produce any physical discomforts? And what's more, I think it's naive to think that when a person is experiencing emotional and behavioral issues, and we assume that is due to a problem within the body, that the problem MUST be with the brain. We know if several other issues (e.g. hypothyroidism) that can affect mood.
I guess I'm making a couple of points here. First, I would think that it would be a prerequisite to being considered a science that your field be able to define your subject matter in a specific way that includes everything that you study and excludes those things that are outside its scope (which doesn't mean it has no relationship to other fields, of course, as all things are linked and related to one another).
But most importantly consider this: every time in history, on an individual and collective level, whenever we've discovered the biological source of a person's "mental illness", that person's problem immediately exits the field of psychiatry and enters another field of medicine. If it's discovered that they have a brain tumor or Ms, they are sent to neurology. If they have syphilis, they are sent to infectious disease specialists, etc. What do you think would happen, hypothetically, if it were discovered that schizophrenia were a legit brain disease, or that bipolar were an endocrine problem? Would psychiatrists still be responsible for treating them?
By the way, if you were to ask me what the subject matter of psychiatry is based on how it is actually practiced presently and historically I would answer with one word: conflict. Psychiatry is the study and "treatment" of inter- and intra-personal conflict.