 According to Stumpf, the cost for adhering to the native theory is nothing less than to abandon the very principles of descriptive psychology and phenomenology. Moreover, besides the difficulties mentioned by Stumpf in this passage, Gestalt psychology conveys philosophical presuppositions which are at the heart of the dispute. The first is a form of sensual naturalism that Stumpf rejects. The second assumption concerns the metaphysical question of psychophysical relations in Köhler’s work and the attempt to introduce the concept of Gestalt in the field of physics. Stumpf believes that Köhler’s isomorphism thesis, which postulates the existence of an identity relation between the structural laws of the psychical world and that of the physical world, presupposes the theory of psychophysical parallelism, which Stumpf rejects as well.