The article aims to unfold this fundamental thesis of Carl Fortlage’s psychology. In the first part, it will provide a brief introduction to his life and work in order to situate the psychology in his oeuvre as a whole. The main section, divided into various subsections, will develop the essential features of Fortlage’s System der Psychologie, which render the above thesis comprehensible. Subsequently, two of the central theorems will be systematically deepened and philosophically-historically traced back to late antiquity, especially to Synesius of Cyrene, with whom Fortlage is familiar. A conclusion with a view to Fortlage’s significance for contemporary introspection research concludes the article.