"Beauty in itself," is simply a word, it is not even a concept. In the beautiful, man postulates himself as the standard of perfection
 Friedrich Nietzsche, Twilight of the Idols (1889). copy citation

add
Author Friedrich Nietzsche
Source Twilight of the Idols
Topic perfection postulate
Date 1889
Language English
Reference
Note Translated by Anthony M. Ludovici
Weblink https://www.gutenberg.org/files/52263/52263-h/52263-h.htm

Context

“Beautiful and Ugly:—Nothing is more relative, let us say, more restricted, than our sense of the beautiful. He who would try to divorce it from the delight man finds in his fellows, would immediately lose his footing. "Beauty in itself," is simply a word, it is not even a concept. In the beautiful, man postulates himself as the standard of perfection; [Pg 75] in exceptional cases he worships himself as that standard. A species has no other alternative than to say "yea" to itself alone, in this way. Its lowest instinct, the instinct of self-preservation and self-expansion, still radiates in such sublimities.” source