Abstract
The historical development of the concept of psychosis and its increasing differentiation from the neuroses up to the modern classification systems, Diagnostic and Statistical Manual of Mental Disorders and International Statistical Classification of Diseases, is initially presented. In portraying this development, the struggle surrounding the clinical relevance of concepts on the one hand and their reliability and validity on the other are reflected. Thus far, diagnostic reliability has primarily been improved by focusing on externally observable symptoms in connection with expression and behavior. The identification of disease-specific symptoms, however, is principally achieved through the differential description of subjective experience. How this experience is to be explored and assessed remains for the most part unclear. With reference to its founder Karl Jaspers, the phenomenological method is presented as the decisive instrument for the assessment of experience. It is shown that a return to the legacy of phenomenology and a reformulation of the long-standing question concerning the specific symptoms of the schizophrenic psychosis are currently in progress. The revival of historical knowledge and a focus on direct clinical phenomena continue to provide inspiration for further advancement in modern psychiatry.