It has been elsewhere noted that the effect of Kant's philosophy was not dissimilar to that of a successful operation on cataract, and the aim of the "World as Will and Idea" is to place in the hands of those on whom that operation has been satisfactorily performed a pair of such spectacles as are suitable to convalescent eyes. Schopenhauer is therefore in a measure indebted to Kant, as also, it may be added, to Plato, and the sacred books of the Hindus. In saying, however, that Schopenhauer is indebted to Kant, it is well to point out that Schopenhauer begins precisely where Kant left off. Kant's great merit consisted in distinguishing the phenomenon from the thing-in-itself, or in other words, in showing the difference between that which seems and that which is.[8] For the inaccessible thing-in-itself he had no explanation to offer. He called it the _Ding an sich_, regarded it as the result of an unintelligible cause, and then left it to be a bugbear to every student of his philosophy. This unpleasant _Ding an sich_ was exorcised, and well-nigh banished for good and all, by Fichte and Hegel; but Schopenhauer reëstablished the incomprehensible factor on a fresh basis, christened it "Will," and asserted it to be the creator of all that is, and at once independent, free, and omnipotent; in other words, the interior essence of the world of which Christ crucified is the sublime symbol. Thus disposed of, the _Ding an sich_ may now be left to take care of itself, and the examination of the great theory begun. Schopenhauer opens his philosophy with the formula, "The world is my idea;" a formula which, it may be noted, condenses in the fewest possible words all that is worth condensing of the idealism of Germany. Beginning in this manner it is evident that he proposes to show neither whence the world comes nor whither it tends, nor yet why it is, but simply, _what it is_. The question has been asked before. According to Schopenhauer, the world is made up of two zones, the real and the ideal; and it may here be said that over the real and the ideal Schopenhauer successfully read the banns. To return, however, to the opening formula. "The world is my idea" is a truth which holds good for everything that lives and thinks, but which, however, is appreciable only by man. When appreciated, it is at once clear that what we know is neither a sun nor an earth, for we have at best an eye which sees the one, and a hand which feels the other. In brief, we are unacquainted with either forms or colors; we have but senses which represent them to us, while objects exist for us merely through the medium of the intelligence. Indeed, as Schopenhauer has said, no other truth is more certain and less in need of proof than this,--that the whole world is simply the perception of a perceiver; in a word, idea.