Since 1889 he served as the Director of the Royal Teacher's School. In 1894 he was appointed as a professor extraordinarius, and since 1896 as the full professor of theoretical and practical philosophy and pedagogy at the Faculty of Philosophy in Zagreb. He retired in 1923. He was the dean of the Faculty of Arts in the periods of 1898–1899, and 1912–1913, and also the rector of the University of Zagreb in the period 1899–1900. He was appointed as a corresponding member of the Yugoslav Academy of Sciences and Arts in 1891, and as a full member in 1899. He served as the president of Matica hrvatska in 1902–1909. In 1892 he became an honorary member of the Croatian Pedagogic-Literary Society, in 1924 a member of the Society of the Brothers of the Croatian Dragon, and an honorary citizen of Krapina.
As a philosopher, he was influenced by Leibniz, Johann Friedrich Herbart and Hermann Lotze, representing the "spiritualist positivism" movement and the view that the knowledge of the world can be reached only by the combined efforts of science, art and religion. He wrote on the fundamental questions of philosophy, which he defined as the science of "final causes and purposes of being". In the treatise Zadnja bića he conceived a world composed from the variety of simple, immutable soullike "last beings" that are connected by the feeling of touch and hierarchically arranged according to different levels of awareness, with absolute consciousness, the God, occupying the peak of the hierarchy. According to Arnold, faith represents the supreme stronghold of a man and the basic principle of harmony of his spiritual functions. According to some opinions, in his later works such as O psihologiji bez duše and Monizam i kršćanstvo Arnold has abandoned his earlier philosophical views. Others such as Pavao Vuk-Pavlović, Stjepan Matičević and Blaženka Despot however maintain that Arnold's philosophical view was uniform and consistent - spiritualist pluralism which doesn't completely exclude spiritualist monism. Arnold is one of the founders of the Croatian philosophical terminology, having authored two influential high school textbooks that were standard textbooks for more than thirty years. At Arnold's proposal, in 1896 the Pedagogical Seminar for theoretical and practical training of future secondary school teachers was founded at the Faculty of Philosophy in Zagreb. As a teacher, organizer of educational system, the first Professor of Pedagogy at the Faculty of Philosophy and the first head of the Pedagogical Seminar, has influenced many generations of teachers. Arnold died in Zagreb.
Works
In literature:
Izabrane pjesme, 1899, Zagreb
Čeznuća i maštanja i pjesme, 1900–1907, Zagreb
S visina i dubina, 1918, Zagreb
Izabrane pjesme, 1923, Zagreb
Na pragu vječnosti, 1935, Zagreb
In philosophy:
Etika i poviest, 1879, Zagreb
Logika za srednja učilišta, 18881, 19235, Zagreb
Psihologija za srednja učilišta, 18931, 19237, Zagreb