 The starting point of Stumpf’s classification is the cardinal distinction made in Brentano’s Psychology from an Empirical Standpoint between two classes of phenomena, physical phenomena and mental phenomena, or what Stumpf calls sensory phenomena and psychical functions. (R. Rollinger, 2008b) The domain of psychical functions, to which internal perception gives access, belongs to descriptive psychology which constitutes the foundation of one of the two branches of Stumpf’s classification of sciences, namely the human sciences (Geisteswissenschaften). Human sciences such as aesthetics and ethics, are closely related to the major classes of the simple functions such as the class of emotions in ethics, for instance, and are defined as the sciences of complex functions. (Stumpf, 1906a, p. 21) The other branch is that of the natural sciences and it is based on physics. In the perspective of Stumpf’s Erfahrungsphilosophie, physics takes as its starting point the phenomena of external perception, and its task is to explain the regularity of phenomena by using the laws of physics. But Stumpf is not a phenomenalist as he advocates instead a form of critical realism that he conceives as the antithesis of phenomenalism such as that which he attributes to classical empiricism and to E. Mach. Nevertheless, Stumpf agrees with the latter that all concepts, including the concept of causality, originate from the contents of sensory experience and are the result of a process of abstraction. However, Stumpf clearly distinguishes the question of the origins of concepts, belonging to descriptive psychology, from the question of the origins and justification of knowledge, which falls within the theory of knowledge. This distinction is at the heart of his habilitation thesis and we shall have the opportunity to return to this important issue.