Emil Kraepelin is well known due to his development of the psychiatric classification. The ICD-10 and DSM-IV classification is based on the dichotomy of endogenous psychoses into affective psychoses and schizophrenia as early as 1899. Moreover, beside his classification system he put enormous impact on the development of psychiatry to an empirical field of science. The research activities of Kraepelin and his coworkers show that he was not only the most active researcher in the field of psychiatry in his time but also that his research activities included a lot of clinical and experimental work in different disciplines of psychiatry, including psychology, pharmacology and natural sciences as 'Hilfswissenschaften'. Due to his extraordinary position also in his time he brought together important researchers of this time, in particular after the foundation of a psychiatric research institute. Alois Alzheimer, Franz Nissl, Robert Gaupp, or Korbinian Brodman are only a few of his well known coworkers. Kraepelin tried to bring foreward the empirical knowledge in psychiatry, he did not want to have cessation in psychiatry in general and in the classification of psychiatric disorders in particular. He discussed and partly revisted his view and his theoretical approach in the different editions of his textbook according to the state of his empirical knowledge. This is also true for the dichotomy. More than twenty years after the 6th edition of his textbook, he wrote in an essay 'Die Erscheinungsformen des Irreseins' ('The manifestations of insanity') regarding the dichotomy: "No experienced diagnostician would deny that cases where it seems impossible to arrive to a clear decision, despite extremely careful observation, are unpleasantly frequent." and "....therefore, the increasingly obvious impossibility to separate the two respective illnesses satisfactorily should raise the suspicion that our question is wrong". This contribution shows that Kraepelin himself questioned his dichotomy of dementia praecox and manic depressive insanity, a discussion which is lively still today--more than 80 years later.