Except it is noticed anecdotally and proven statistically for many people: 1. If somebody is depressed given everything is good in their life – you give them antidepressants – they get better. 2. Somebody thinks the CIA is spying on him (and nobody is really spying on them) – you give them antipsychotics – they kind of stop thinking that CIA is following them. Also as a doctor you probably aware that no illness is verified by a lab test. It takes a physician, symptoms and complaints, then a lab test is appointed then only diagnose confirmed. The whole medicine works on Bayesian reasoning. Yes, there is no lab test currently for mental illness (there are many pieces of evidence of brain pathology involved in the case of schizophrenia or bipolar or borderline). But read the history of medicine – there were no diagnostic tests for many autoimmune diseases for many years and even now there are plenty of diseases without an unequivocal test. In medicine, many diseases were discovered just by autopsy. So for many diseases, physicians do the diagnostics but then use the treatment response as confirmation of their reasoning.