It is not surprising that we can find many similarities among the students of Franz Brentano, not only in the development of their philosophical positions, but also sometimes in their personal lives. For instance, both Anton Marty and Carl Stumpf first both joined the seminary and then both quit it again when Brentano left the church following the controversy on papal infallibility. Such parallelisms can also be found in the early careers of Edmund Husserl and Christian von Ehrenfels. Both studied under Brentano, were influenced by Alexius Meinong’s works on the theory of relations and both, inspired by Ernst Mach, developed a philosophical notion of Gestalt around 1890. They also both wrote on the philosophy of mathematics: Ehrenfels’ 1885 dissertation Größenrelationen und Zahlen: eine psychologische Studie (Relations of Magnitude and Numbers: a Psychological Study) and Husserl’s 1887 habilitation work Über den Begriff der Zahl. Psychologische Analysen (On the Concept of Number. Psychological Analyses) and then both in 1891 Ehrenfels’ article “Zur Philosophie der Mathematik” (“On the Philosophy of Mathematics”) and of course Husserl’s book Philosophie der Arithmetik. Psychologische und Logische Untersuchungen (Philosophy of Arithmetic. Psychological and Logical Investigations).
We know that Ehrenfels did read Husserl’s On the Concept of Number, because he refers to it twice in his 1891 article. However, according to the name index in the four volumes of Ehrenfels Philosophische Schriften, that’s all the references to Husserl you get in his works. We also know that Husserl read Ehrenfels’ article, because his copy at the Husserl-Archives is annotated. Beyond that, Husserl hardly ever refers to Ehrenfels in his eary works and then almost exclusively to distinguish their respective developments of the notion of Gestalt. The one single mention of Ehrenfels’ name in the Husserl-Chronik is merely to report the bare fact that Husserl acquired Ehrenfels’ article on the philosophy of mathematics in August 1891. The further entries in Schuhmann’s Index Nominum, as far as I have been able to verify, also contain nothing of substance.
However, the truly surprising thing is that they apparently never exchanged any letters. In the first volume of Husserl’s Briefwechsel, containing his correspondence with the members of the School of Brentano, there is not a single letter between Husserl and Ehrenfels. According to the Website of the Forschungsstelle und Dokumentationszentrum für Österreichische Philosophie, which besides harboring the Brentano-Archives also has an extensive collection of manuscripts and letters of many other austrian philosophers, there is also not a single letter between Ehrenfels and Husserl.
Even though they both worked on the same topics in the same period, in very similar contexts and based by and large on the same sources (Brentano’s lectures, Meinong’s theory of relations, Kerry’s articles, Mach’s book, etc.), they never truly engaged with one another directly, though they must have certainly been aware of their respective interests and endeavors.