Full Idea
That forgetting exists has never yet been demonstrated, but only that many things do not occur to us when we want them to.
Gist of Idea
There is no proof that we forget things - only that we can't recall
Source
Friedrich Nietzsche (Unpublished Notebooks 1881-82 [1882], 12[1]123)
A Reaction
There is now quite a lot of evidence that there innumerable memories buried in that mind that we seem unable to directly recall. He is right that we can hardly demonstrate this negative fact.
Book Reference
Nietzsche,Friedrich: 'The Joyful Science, and 1881-82 fragments (v 6)', ed/tr. Del Caro,Adrian [Stanford 2023], p.352