Deficiency of understanding we call stupidity : deficiency in the application of reason to practice we shall recognise later as foolishness : deficiency of judgment as silliness , and lastly, partial or entire deficiency of memory as madness.
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

add
Author Arthur Schopenhauer
Source The World as Will and Representation
Topic foolishness stupidity
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

“We cannot, however, discuss this here; it will find its proper place in the second book, when we consider the harmony or so-called teleology of nature: and the 27th chapter of the supplementary volume is expressly devoted to it.
Deficiency of understanding we call stupidity : deficiency in the application of reason to practice we shall recognise later as foolishness : deficiency of judgment as silliness , and lastly, partial or entire deficiency of memory as madness. But each of these will be considered in its own place. 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.” source