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.
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

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Author Arthur Schopenhauer
Source The World as Will and Representation
Topic illusion comprehension
Date 1819
Language English
Reference
Note Translated by R. B. Haldane and J. Kemp
Weblink http://www.gutenberg.org/files/40097/40097-h/40097-h.html

Context

“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.” source