Karl Leonhard has presented a globally recognized masterpiece of psychopathology with "Classification of Endogenous Psychoses and Their Differentiated Etiology," which builds on the works of Carl Wernicke and Karl Kleist. The introduction by Helmut Beckmann emphasizes the significance of Leonhard's findings in light of current research on psychoses. Through meticulous and brilliant psychopathological cross-sectional and longitudinal studies, Leonhard has delineated a wide variety of different endogenous psychoses and made valid statements about their respective prognoses as well as their varying origins due to environmental factors and/or genetics. The accuracy of his descriptions and findings reaches a level of validity that is not even closely matched by current classification systems. It is becoming evident that this differentiated nosological framework opens up new perspectives for research on psychoses.