A forerunner of neo-Kantianism, in his best-known book, Kant und die Epigonen, he deals with the philosophy after Kant, discussing Fichte, Schelling, Hegel, Fries, Herbart and Schopenhauer. Having credited Kant's philosophy (though criticizing it on the vital point of accepting a thing-in-itself), he focuses on what he sees as the shortcomings in the approaches of Kants successors. He frequently ends a section with the statement that one should return to Kant.