Cohen’s long-time co-worker and friend, Paul Natorp, carried out a more concrete, thorough, and insightful examination of ancient and modern philosophy by means of a systematic understanding of the problems. In the autumn of 1881, Natorp habilitated at Marburg University; in 1885 he became a lecturer at the university; in 1893, as successor to J. Bergmann (see below), he became Ordinarius Professor of Philosophy. In 1922 he became Emeritus, in spite of the fact that he was still active with lectures and classes. Natorp died shortly after his 70th birthday in August 1924. Because Natorp’s philosophical work initially took place scrupulously in the same spirit as Cohen’s, he could later see the essential gaps and one-sided aspects of the system more clearly, and he could bring it to a more originally grounded, independent level of development. Natorp’s earliest investigations were concerned with the loosening up of ancient philosophy through the history of [philosophical] problems. The work Forschungen zur Geschichte des Erkenntnisproblems im Altertum (1884) had a strong influence on science. The work Platos Ideenlehre. Eine Einführung in den Idealismus (1903) indeed met with fierce opposition. Regardless of the tenability of individual interpretations, it performed the urgent task of clarifying that the history of Philosophy cannot dispense with a systematic understanding of the problem as a hermeneutical presupposition of its work. In a treatise that is too seldom considered, called Über Thema und Disposition der aristotelischen Metaphysik (Philos. Monatshefte, vol. XXIV, 1888), Natorp anticipated results and problems in which the present age first became more accessible.

The transcendental laying-of-the-ground for logical, ethical and aesthetic conduct has its “highest point” in the subject. Thus the ground-laying itself first comes to the ground through a thematic consideration of consciousness in the sense of a nonempirical Transcendental Psychology. Natorp communicated his first experiments in this direction in his Einleitung in die Psychologie nach kritischer Methode (1888). During the following two decades, in the course of his debate with the more spirited, well-formed, fundamental conception of  psychology (Dilthey and Husserl), the importance of which psychology itself can only grasp today, Natorp pushed forward to a radical formulation of the problem. The new position was in evidence in the re-casting of the earlier Einleitung, which appeared in 1912 as Allgemeine Psychologie nach kritischer Methode I. In this work, as was the case with all of his philosophical work, Natorp increasingly aimed for a systematic unfolding of the systematic unity of philosophy. In order to overcome a superficial and after-the-fact condensation of the transcendental philosophical disciplines, which had always fundamentally remained standing for Cohen, it was above all a matter of breaking away from the leveling, which Cohen forced, of all possible ways of comportment of Spirit to the sciences of these ways of comportment. With the elimination of the methodical priority of the sciences, theoretical comportment draws “along-side” the “atheoretical,” i.e., the moral, the artistic, and the religious. The idea of logic was freed from the confines of a laying of the grounds for the sciences, that is, from a “theorizing,” and along with the “practical” and the “poetic,” it was pre-classified as universal doctrine of categories. The freer posing of philosophical questions concerning the originality of the individual areas of spiritual life, prepared for in this way, resulted in a more open interpretation of spiritual history. At the same time, it made possible the positive evaluation of the fundamental meaning of a phenomenological categorical analysis of “subjective” and “objective” spirit. Natorp’s own Vorlesungen über praktische Philosophie, which were still being readied for publication and which first appeared after his death (1925), offered a concrete glimpse into the new and comprehensively systematic tendencies in his thought.

From the Philosophical Seminar that was established in 1900 a series of valuable investigations have emerged which, since 1906, have been assembled in Philosophische Arbeiten, edited by H. Cohen and P. Natorp.