Full Idea
The model of a complete fiction is logic. Here a thinking is made up where a thought is posited as the cause of another thought.
Gist of Idea
Logic is a fiction, which invents the view that one thought causes another
Source
Friedrich Nietzsche (Unpublished Notebooks 1885-86 [1886], 34[249])
Book Reference
Nietzsche,Friedrich: 'Fragments from 1885-86 (v 16)', ed/tr. Del Caro,Adrian [Stanford 2020], p.73
A Reaction
He could almost be referring to Frege's Third Realm. Most hard core analytic philosophers seem to think that propositions have tight logical relationships which are nothing to do with the people who think them.