German philosophy
Neo-Kantianism refers broadly to the revival of the type of philosophy explained by Immanuel Kant and of the interpretations of Kant provided by post-Kantian philosophers such as Schopenhauer, Jakob Friedrich Fries and Johann Friedrich Herbart. Major figures in the neo-Kantian movement, which began around the 1860s, include Friedrich Albert Lange and Hermann Cohen.
German idealism
Kant's work purported to bridge the two dominant philosophical schools in the 18th century: 1) rationalism, which held that knowledge could be attained by reason alone "a priori" (prior to experience), and 2) empiricism, which held that knowledge could be arrived at only through the senses "a posteriori" (after experience), as expressed by philosopher David Hume, whom Kant sought to rebut. Kant's solution was to propose that while we can know, via sensory experience, particular facts about the world (which he termed phenomena), we cannot know the "form" they must take prior to any experience (which he called noumena). That is, we cannot know "what" objects we will encounter, but we can know "how" we will encounter them. Kant called his mode of philosophising "critical philosophy", in that it was supposedly less concerned with setting out positive doctrine than with critiquing the limits to the theories we "can" set out. The conclusion he presented, as above, he called "transcendental idealism". This distinguished it from classical idealism and subjective idealism such as George Berkeley's, which held that external objects have actual being or real existence only when they are perceived by an observer. Kant said that there are things-in-themselves, noumena, that is, things that exist other than being merely sensations and ideas in our minds. Kant held in the "Critique of Pure Reason" that the world of appearances (phenomena) is empirically real and transcendentally ideal. The mind plays a central role in influencing the way that the world is experienced: we perceive phenomena through time, space and the categories of the understanding. It is this notion that was taken to heart by Kant's philosophical successors.