Full Idea
The faith in the categories of reason is the cause of nihilism; we have measured the value of the world according to categories that refer to a purely fictitious world.
Gist of Idea
Nihilism results from valuing the world by the 'categories of reason', because that is fiction
Source
Friedrich Nietzsche (The Will to Power (notebooks) [1888], §12B)
Book Reference
Nietzsche,Friedrich: 'The Will to Power', ed/tr. Kaufmann,W /Hollingdate,R [Vintage 1968], p.13
A Reaction
Presumably this refers to Kant, whose dogmatic assertions about the structure of human reason are as open to objection as those of Freud. Nietzsche may have a very profound truth here.