As currently used, the diagnosis of depression includes a wide range of clinical phenomena. This has not always been the case. Near the end of the 19th century, when the term depression began to evolve the meanings that it has today it was applied primarily to psychotics. The formulations of Freud in Mourning and Melancholia (1917), and of Kraepelin in Manic Depressive Insanity (1921) were based upon observations of patients who were both depressed and psychotic. In their work the contrast was between psychotic depression (or “melancholia”) on one hand, and normal sadness on the other. In the succeeding half-century, however, as psychiatry has extended its boundaries, increasing attention has been focused on non-psychotic depressions, often called “neurotic” or “reactive.” As these “neurotic” or “reactive” depressions reached public attention, a debate began over the way in which the depressive population should be described and the extent to which it should be subdivided. Critical and often sarcastic written battles were fought between the separatists and the unifiers during the 1920's and 1930's. These debates have been informatively chronicled by Partridge (1949). We have found it useful to divide these theorists into unifiers, dualists, and pluralists.