 The theory of knowledge occupies a central place in Stumpf’s philosophy in his later works, especially in Erkenntnislehre, and we know that this discipline has always been at the heart of his teaching up to the end of his career in Berlin. We saw that Stumpf associates it both with the neutral sciences and the foundation of knowledge, specifically with the ultimate justification of knowledge. (Stumpf, 1939–1940, p. 7, 124) One of the central questions of the theory of knowledge is psychologism, which is the main topic in Stumpf’s article “Psychology and the Theory of Knowledge.” (D. Münch, 2002; G. Fréchette, 2013) Stumpf advocates a form of critical realism that he specifically opposes to Kantian idealism, and which maintains that the outside world is always the object of a supposition or hypothesis. Moreover, he claims that the only material that we have for formulating such an hypothesis are the concepts drawn from experience that enable us “to calculate the course of events.” One of the main tasks of the theory of knowledge, therefore, is to account for the most general modes of knowledge of the outside world. (Stumpf, 1891, p. 503) Stumpf distinguishes the theory of knowledge as a field of study from that of psychology on the basis of the cardinal distinction between the domain of concepts and that of propositions or necessary truths, as elaborated in his habilitation thesis.