In so doing, he started by indirectly conceptually reflecting on the conditions that exist in the observing subject that make possible verbal judgments about objective experience.
...the solution of the riddle of the world is possible only through the proper connexion of outer with inner experience...The fundamental principles of knowledge cannot be transcendentally explained or proved, they can only be immediately, directly known.
Guyer stated that Schopenhauer raised important questions regarding the possibility of Kant's transcendental arguments and proofs.
Also, the distinction between logical and real relations, as well as the difference between phenomena and things-in-themselves, played an important role in Schopenhauer's philosophy.
Where Kant was analyzing the conceptual conditions that resulted in the making of verbal judgments, Schopenhauer was phenomenologically scrutinizing intuitive experience.
In one case, though, it is claimed that Schopenhauer raised a very important criticism: his objection to Kant's assertion that a particular event can be known as being successive only if its particular cause is known.
In philosopher Derek Parfit's 2011 book On What Matters, Volume 1, Parfit presents an argument against psychological egoism that centers around an apparent equivocation between different senses of the word "want": Michael Kelly, in the preface to his 1910 book Kant's Ethics and Schopenhauer's Criticism, stated: "Of Kant it may be said that what is good and true in his philosophy would have been buried with him, were it not for Schopenhauer...." Immanuel Kant himself predicted a response to Schopenhauer's argument that he redundantly repeated the ancient command: "don't do to another what you don't want done to you", that is, the Golden Rule, and famously criticized it for not being sensitive to differences of situation, noting that a prisoner duly convicted of a crime could appeal to the golden rule while asking the judge to release him, pointing out that the judge would not want anyone else to send him to prison, so he should not do so to others.
Also, it is exactly for being cold and dead because it is to be followed without love, feeling, or inclination, but merely out of a sense of duty, both in the theory and in its practice, that the Categorical Imperative is absolute, metaphysical and moral.