Now as Kant’s separation of the phenomenon from the thing in itself, arrived at in the manner explained above, far surpassed all that preceded it in the depth and thoughtfulness of its conception, it was also exceedingly important in its results. For in it he propounded, quite originally, in a perfectly new way, found from a new side and on a new path, the same truth which Plato never wearies of repeating, and in his language generally expresses thus: This world which appears to the senses has no true being, but only a ceaseless becoming; it is, and it is not, and its comprehension is not so much knowledge as illusion. This is also what he expresses mythically at the beginning of the seventh book of the Republic, the most important passage in all his writings, which has already been referred to in the third book of the present work. He says: Men, firmly chained in a dark cave, see neither the true original light nor real things, but only the meagre light of the fire in the cave and the shadows of real things which pass by the fire behind their backs; yet they think the shadows are the reality, and the determining of the succession of these shadows is true wisdom.
The same truth, again quite differently presented, is also a leading doctrine of the Vedas and Puranas, the doctrine of Mâyâ, by which really nothing else is understood than what Kant calls the phenomenon in opposition to the thing in itself; for the work of Mâyâ is said to be just this visible world in which we are, a summoned enchantment, an inconstant appearance without true being, like an optical illusion or a dream, a veil which surrounds human consciousness, something of which it is equally false and true to say that it is and that it is not.
But Kant not only expressed the same doctrine in a completely new and original way, but raised it to the position of proved and indisputable truth by means of the calmest and most temperate exposition; while both Plato and the Indian philosophers had founded their assertions merely upon a general perception of the world, had advanced them as the direct utterance of their consciousness, and presented them rather mythically and poetically than philosophically and distinctly. In this respect they stand to Kant in the same relation as the Pythagoreans Hicetas, Philolaus, and Aristarchus, who already asserted the movement of the earth round the fixed sun, stand to Copernicus.
Such distinct knowledge and calm, thoughtful exposition of this dream-like nature of the whole world is really the basis of the whole Kantian philosophy; it is its soul and its greatest merit. He accomplished this by taking to pieces the whole machinery of our intellect by means of which the phantasmagoria of the generally, are not to be applied to deduce and explain existence itself that thus the validity of these laws is only relative, i.e., only arises after existence; the world of experience in general is already established and present; that consequently these laws cannot be our guide when we come to the explanation of the existence of the world and of ourselves.
Schopenhauer, Arthur. Delphi Collected Works of Arthur Schopenhauer (Illustrated) (Delphi Series Eight Book 12) (pp. 548-549). Delphi Classics. Kindle Edition.