German idealism
Neo-Kantianism refers broadly to a revived type of philosophy along the lines of that laid down by Immanuel Kant in the 18th century, or more specifically by Schopenhauer's criticism of the Kantian philosophy in his work "The World as Will and Representation" (1818), as well as by other post-Kantian philosophers such as Jakob Friedrich Fries and Johann Friedrich Herbart. It has some more specific reference in later German philosophy.
Otto Liebmann
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.