The assumption that a core group of depressive disorders is due to a discontinuous change in the function of the brain system, suggests that the symptoms of the core group of depressive disorders should be discerned from those of non‒core depression. Core depression is thought to correspond to depression of an endogenous nature, which has recently been disregarded in diagnosing mood disorders. However, in diagnosing endogenous depression, we can identify its characteristic symptoms by referring to a traditional symptomatology. Therefore, the idea of Verstehen（Jaspers, K）becomes essential, but has been neglected in academic journals in English‒speaking countries. The depressive mood in endogenous depression may be an inhibition of various kinds of emotion, which can never be experienced within a normal emotional state. Thus, it is thought to be‘unverstaendlich’（incomprehensible）in nature. The“anhedonia hypothesis”of depression from DSM‒Ⅳ permits the inclusion of the non‒core group of depression into major depressive disorder, because patients with the endogenous type suffer not only from a loss of pleasure but also from a loss of sadness. A new type of depression that has recently been debated in Japan is diagnosed as major depressive disorder in DSM‒Ⅳ, but many suspected cases are thought to be due to a psychogenic state or neurotic condition, because their symptoms are thought to be‘verstaendlich’（comprehensible）.
 Endogenous depression can manifest as manic‒depressive disorder in almost all cases, but psychiatric practitioners have taken the necessary precautions against the risk of a shift to a manic state even when treating pure depression which seemingly appears to have no manic component. According to recent studies on bipolar disorder, pure mania is not empirically found. Thus, the manic pole may not exist, and we may be able to think of endogenous depression as manic‒depressive disorder based on its genetic entity. Additionally, from the viewpoint of its symptomatology there is one pole of depression modified by manic tendencies based on various intensities. The current study presents the hypothesis that there is one entity for disease, manic‒depression, but whether the disorder manifests itself as uni‒ or bipolar disorder depends upon the regulation of manic manifestations by a meticulous, obsessive‒compulsive personality which has been considered to be the premorbid personality for unipolar depression.