I am reading "The World as Will and Representation" by Arthur Schopenhauer (Norman, J., Welchman, A., & Janaway, C. (Eds.). (2010). Schopenhauer: 'The World as Will and Representation'). In the Appendix of the book titled "Criticism of Kantian philosophy", he writes:
Now I return to Kant’s great error touched on above, the fact that he did not properly distinguish intuitive from abstract cognition, which gave rise to a fatal confusion that we must consider more closely. If he had clearly distinguished intuitive representations from concepts thought merely in the abstract...
I am struggling to understand what Schopenhauer means by "abstract cognition and intuitive cognition". What is the meaning of those two terms in the Kantian framework? And why does Schopenhauer thinks that Kant failed to acknowledge the distinction between the two?