Full Idea
Kant's greatest merit is the distinction of the phenomenon from the thing-in-itself.
Gist of Idea
Kant rightly separates appearance and thing-in-itself
Source
Arthur Schopenhauer (The World as Will and Idea [1819], I 417 App), quoted by Peter B. Lewis - Schopenhauer 3
Book Reference
Lewis, Peter B.: 'Schopenhauer' [Reaktion Books 2012], p.63
A Reaction
This is Schopenhauer firmly opposing the Absolute Idealism of Kant's successors, who dismissed the 'thing-in-itsef'.