Error is opposed to truth , as deception of the reason : illusion is opposed to reality , as deception of the understanding.
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

add
Author Arthur Schopenhauer
Source The World as Will and Representation
Topic reality illusion
Date 1819
Language English
Reference
Note Translated by R. B. Haldane and J. Kemp
Weblink http://www.gutenberg.org/files/38427/38427-h/38427-h.html

Context

“That which is correctly known by reason is truth , that is, an abstract judgment on sufficient grounds (Essay on the Principle of Sufficient Reason, § 29 and following paragraphs) ; that which is correctly known by understanding is reality , that is correct inference from effect on the immediate object to its cause. Error is opposed to truth , as deception of the reason : illusion is opposed to reality , as deception of the understanding. The full discussion of all this will be found in the first chapter of my essay on Light and Colour. Illusion takes place when the same effect may be attributed to two causes, of which one occurs very frequently, the other very seldom; the understanding having no data to decide which of these two causes operates in any particular case,—for their effects are exactly alike,—always assumes the presence of the commoner cause, and as the activity of the understanding is not reflective and discursive, but direct and immediate, this false cause appears before us as a perceived object, whereas it is merely illusion.” source