The categories used in psychiatric diagnosis are based on observation of signs and symptoms, rather than on pathological processes. One can make use of a few signs, such as facial expressions associated with depression or the flight of ideas associated with mania. But what clinicians mainly use for diagnosis are symptoms, the subject experiences reported by patients. Psychiatrists have little knowledge of the processes that lie behind these phenomena. Thus, psychiatric diagnoses, with very few exceptions, are syndromes, not diseases.

The Intelligent Clinician's Guide to the DSM-5