 The position adopted by Stumpf in this debate consists in conceding to criticism that necessary truths are irreducible to facts while admitting, with psychologists, that psychology is essential to the theory of knowledge. Hence the main mistake he attributes to Kantianism, which consists in refusing the assistance of psychological research in the theory of knowledge. (Stumpf, 1891, p. 493, 500) The field of psychology is understood here in a sense that is broad enough as to include sensory phenomena and mental functions because, in 1891, Stumpf did not explicitly distinguish the field of phenomenology from that of descriptive psychology. Now the main criticism he addresses to Kant focuses on the dichotomy between form and matter, and it is primarily the Kantian doctrine of phenomena (the manifold of intuition) that he holds responsible for its most obvious “mistakes.” In this article, Stumpf reiterates the main criticism that he addressed to Kant in his Raumbuch, namely “the doctrine of space as a subjective form and the Kantian conception of sensations as amorphous and unstructured matter designed to support the synthetic and unifying activity of the understanding.” (Stumpf, 1873, p. 15 f.)