His major written work is "Philosophie der Arithmetik" (1891). In these first works he tries to combine mathematics, psychology and philosophy with a main goal to provide a sound foundation for mathematics. He analyzes the psychological process needed to obtain the concept of number and then tries to build up a systematical theory on this analysis. To achieve this he uses several methods and concepts taken from his teachers. From Weierstrass he derives the idea that we generate the concept of number by counting a certain collection of objects. From Brentano and Stumpf he takes over the distinction between "proper" and "improper" presenting.