All diagnostic systems depend upon classification. What function does classification serve? Imagine if we had no common, standardized classification system for mental illness. With no common language to describe clinical observations, there could be no coordination among clinicians, researchers, or people working in public policy. There would be no way to research the prevalence, etiology (cause), outcome, or progression of the illness. Without a grounding of scientific data, there would be no way to systematically develop and test treatments. Treatments would be fragmented, ad hoc and untested, ultimately based on personal opinion instead of scientific fact.