Full Idea
If we do not know what knowledge is, we cannot possibly answer the question of whether there is knowledge.
Gist of Idea
We can't know whether there is knowledge if we don't know what it is
Source
Friedrich Nietzsche (The Will to Power (notebooks) [1888], §530)
Book Reference
Nietzsche,Friedrich: 'The Will to Power', ed/tr. Kaufmann,W /Hollingdate,R [Vintage 1968], p.286
A Reaction
Obviously Nietzsche is pessimistic about the prospects here, but this is a motto for the whole modern analysis of knowledge, and (besides) we have lots of things (like a concept of identity) which we can't define.