 Philosophy of mind or what is called descriptive psychology represents one of the main axes of Stumpf’s program in philosophy. His treatises in this domain are quite diversified, ranging from animal and developmental psychology to acoustic psychology and Gestalt psychology. Other than his Raumbuch, the book that made Stumpf renowned in psychology is Psychology of Sound (published in two volumes in 1883 and 1890) which was widely received in Germany and England (see reviews by P. Natorp, A. Meinong, T. Lipps and J. Sully). In this book, as in most of his treatises on psychology, Stumpf’s approach is similar to that of Brentano’s descriptive psychology. It differs from experimental and physiological psychology in that it conceptually analyses and describes what physiological psychology explains through the descriptive apparatus of the natural sciences such as biology or physiology. Between these two branches of psychology there is a form of division of labor wherein the first analyses and describes what the other explains. However, descriptive psychology has a methodological priority over experimental psychology in that the analysis and description of the explanandum is a prerequisite to its explanation. The two main tasks assigned to descriptive psychology are the analysis and the classification of mental states (also called acts or mental functions). In his influential study entitled “Phenomena and Mental Functions,” Stumpf shows how phenomena are closely linked in that they form a real unity, which can only be separated by abstraction. He conceives of phenomena as unitary wholes and not as aggregates, and claims that they are perceived as concrete and unitary wholes. As such, phenomena are the contents of sensations that are characterized by certain properties or attributes, for example extension, intensity, brightness, etc., and these contents of sensations are first-order phenomena as opposed to second-order phenomena like “mnemonic images” and phenomena such as color or sound, to the extent that they “are merely represented.” Although Stumpf rarely uses the concept of intentionality, one might say that mental functions are distinct from physical phenomena in that they are related to their objects by intentional relations.