Full Idea
The strength of knowledge does not depend on its degree of truth but on its age, on the degree to which it has been incorporated, in its character as a condition of life.
Gist of Idea
The strength of knowledge is not its truth, but its entrenchment in our culture
Source
Friedrich Nietzsche (The Gay (Joyful) Science [1882], §110)
Book Reference
Nietzsche,Friedrich: 'The Gay Science', ed/tr. Kaufmann,Walter [Vintage 1974], p.169
A Reaction
This seems to be the rather modern idea (in Foucault, perhaps) of knowledge as a central component of culture, rather than as an eternal revelation of facts. Note that he is talking about its 'strength', not its veracity or degree of support.