“Illness is always an interaction between [mind and body]. It can begin in the mind and affect the body, or it can begin in the body and affect the mind, both of which are served by the same bloodstream. Attempts to treat most mental diseases as though they were completely free of physical causes and attempts to treat most bodily diseases as though the mind were in no way involved must be considered archaic in the light of new evidence about the way the human body functions.”