Sous la supervision de Stumpf, il écrivit Ã ber den Begriff der Zahl -LRB- On the concept of Number -RRB- in 1887, which would serve later as the base for his first important work, Philosophie der Arithmetik -LRB- 1891 -RRB-.
Il écrit principalement Philosophie der Arithmetik -LRB- 1891 -RRB-.