Arthur Schopenhauer
Immanuel Kant

The importance of Kant for Schopenhauer, in philosophy as well as on a personal level, cannot be overstated. Kant's philosophy was the foundation of Schopenhauer's, and he had high praise for the Transcendental Aesthetic section of Kant's Critique of Pure Reason. Schopenhauer maintained that Kant stands in the same relation to philosophers such as Berkeley and Plato, as Copernicus to Hicetas, Philolaus, and Aristarchus: Kant succeeded in demonstrating what previous philosophers merely asserted.

Schopenhauer writes about Kant's influence on his work in the preface to the second edition of The World as Will and Representation:

I have already explained in the preface to the first edition, that my philosophy is founded on that of Kant, and therefore presupposes a thorough knowledge of it. I repeat this here. For Kant's teaching produces in the mind of everyone who has comprehended it a fundamental change which is so great that it may be regarded as an intellectual new-birth. It alone is able really to remove the inborn realism which proceeds from the original character of the intellect, which neither Berkeley nor Malebranche succeed in doing, for they remain too much in the universal, while Kant goes into the particular, and indeed in a way that is quite unexampled both before and after him, and which has quite a peculiar, and, we might say, immediate effect upon the mind in consequence of which it undergoes a complete undeception, and forthwith looks at all things in another light. Only in this way can any one become susceptible to the more positive expositions which I have to give. On the other hand, he who has not mastered the Kantian philosophy, whatever else he may have studied, is, as it were, in a state of innocence; that is to say, he remains in the grasp of that natural and childish realism in which we are all born, and which fits us for everything possible, with the single exception of philosophy.

In his study room, one bust was of Buddha, the other was of Kant. The bond which Schopenhauer felt with the philosopher of Königsberg is demonstrated in an unfinished poem he dedicated to Kant (included in volume 2 of the Parerga):

With my eyes I followed thee into the blue sky, And there thy flight dissolved from view. Alone I stayed in the crowd below, Thy word and thy book my only solace.— Through the strains of thy inspiring words I sought to dispel the dreary solitude. Strangers on all sides surround me. The world is desolate and life interminable.

Schopenhauer dedicated one fifth of his main work, The World as Will and Representation, to a detailed criticism of the Kantian philosophy.

Schopenhauer praised Kant for his distinction between appearance and the thing-in-itself, whereas the general consensus in German idealism was that this was the weakest spot of Kant's theory, since, according to Kant, causality can find application on objects of experience only, and consequently, things-in-themselves cannot be the cause of appearances. The inadmissibility of this reasoning was also acknowledged by Schopenhauer. He insisted that this was a true conclusion, drawn from false premises.
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