In the late 1800’s, the German psychiatrist Emil Kraepelin was unhappy with the primitive state of psychiatric diagnosis. So he set up shop at the Heidelberg Clinic and proceeded to study hundreds of patients longitudinally. He recorded his best guess of the initial diagnoses, stashing them in his "diagnosis box," and then revising them after studying his patients more intensively. In 1883, Kraepelin began recording the results of this research in a famous "Textbook of Psychiatry". By carefully tracking the psychiatric symptoms of his patients, he concluded that all non-organic psychotic disorders appeared to fit into two broad categories: "dementia praecox" (corresponding to current day “schizophrenia”) and "manic-depressive insanity," encompassing all of what we know today as both unipolar depression and bipolar disorder. The 6th edition of Kraepelin's textbook (1899) formed much of the basis for another celebrated diagnostic manual of our time: DSM-III.
Source: Edward Shorter, A History of Psychiatry, New York: John Wiley and Sons, Inc., 1997.