Simplify the following text for a Grade 6 native English-speaking person.
Original: There under Stumpf 's supervision he wrote Ã ber den Begriff der Zahl ( On the concept of Number ) in 1887 , which would serve later as the base for his first important work , Philosophie der Arithmetik ( 1891 ) .
Simplified: His major written work is Philosophie der Arithmetik ( 1891 ) .