This article discusses the theoretical backgrounds of E. Kraepelin's clinical nosology. In accordance with his project of a scientific psychiatry, Kraepelin tried to corroborate his clinical categories by scientific means. The scientific theory used was Wundt's experimental psychology, together with 19th-century neurophysiology. This 'psychophysics', however, gradually lost its predominance, and was replaced by the theory of degeneration. Kraepelin failed in connecting psychophysics and the theory of degeneration into an all comprising biological theory. Considering the anti-scientific nature of the latter, this implied the bankruptcy of this first project of a scientific psychiatry.