Full Idea
Appearance as I understand it is the actual and single reality of things - that which first merits all existing predicates.
Gist of Idea
Appearance is the sole reality of things, to which all predicates refer
Source
Friedrich Nietzsche (Unpublished Notebooks 1885-86 [1886], 40[53])
Book Reference
Nietzsche,Friedrich: 'Fragments from 1885-86 (v 16)', ed/tr. Del Caro,Adrian [Stanford 2020], p.208
A Reaction
This is the view espoused by John Stuart Mill (a fact which would shock Nietzsche!). Elsewhere he laughs at the concept of the thing-in-itself as a fiction.
Related Idea
Idea 3583 External objects are permanent possibilities of sensation [Mill]