We know that logic is closely related to Stumpf's theory of relations and eidology, central elements in both his philosophical and empirical works.
*21 presents the "General Theory of Relations" (the theory of relations in extension; in contemporary logic these are treated as sets of ordered pairs, following Wiener).
Relations form the group of objects whose study belongs to the general theory of relations.
This aspect of the general theory of relations is comparable to Husserl's pure logic.
Summing up, Schröder constructed an algebraic version of modern predicate logic and also a theory of relations.
De Morgan gets credit for introducing the concept of relation, but Peirce is considered the true creator of the theory of relations (see, e.g., Tarski 1941: 73).
Stumpf claims that they represent three different fields of research, corresponding to three distinct sciences: phenomenology as the science of phenomena, eidology which studies formations, and the general theory of relations understood as the science of relations as a whole.
More precisely his contribution to logic in these two research areas concerns in fact the specific content of judgment (belief) or what he calls states of affairs, and that part of the theory of relations pertaining to part-whole relations.
One question had been answered by Arwin Korselt (1864 1947), namely there were first-order sentences in the theory of binary relations that were not equivalent to an equation in the calculus of relations thus the calculus of relations definitely had a weaker expressive power than the first-order theory of relations.
In particular, Ehrenfels' teacher Alexius Meinong had dedicated a work specifically to the theory of relations (Meinong 1882), which is also repeatedly referred to by Ehrenfels, underscoring that relations are crucially important to his discussion and explicitly pointing out that relations themselves are nothing but Gestalt qualities (Ehrenfels 1890; Smith 1988, 101).
