 Stumpf focuses particularly in his writings on two specific topics related to the domain of ontology (D. Jacquette, 2000–2001, R. Martinelli, 2015), namely the categories and psychophysical relations. In Erkenntnislehre, he examines the categories of thing (substance), cause, necessity (legality), truth, reality, equality, and number (Stumpf 1939–1940, p. 13) and claims that these Grundbegriffe or categories have their origin in (internal and external) perception. This is primarily the case for the concept of necessity which is at the heart of his critique of psychologism. Stumpf agrees with Hume and J. S. Mill that we cannot observe necessary relations in nature, but he argues that internal perception provides us with many cases of necessary relations such as mathematical axioms, or in the perception of three consecutive sounds where one of them is necessarily intermediate (as to the pitch) between the other two. (Stumpf, 1939, p. 48) The concept of necessity, as the concept of causality, is formed through a process of abstraction and can then be used, hypothetically, to explain natural events based on causal laws. Moreover, we also find in Stumpf an original account of tropes which influenced the young Husserl. (R. Rollinger, 2015)