Like most of Brentano’s students, Ehrenfels also wrote various works on the philosophy of mathematics: first of all his 1885 dissertation on “Größenrelationen und Zahlen: eine psychologische Studie” (“Relations of Magnitude and Numbers: a Psychological Study”), then his article “Zur Philosophie der Mathematik” (“On the Philosophy of Mathematics”) in 1891, the same year as Husserl’s Philosophy of Arithmetic, and finally in 1922 his booklet on Das Primzahlengesetz (The Law of Prime Numbers). As Simons (1988, 10) observes, his early mathematical works are representative of the Brentanian approach as it can also be found in Husserl’s and Stumpf’s habilitation works.