Full Idea
Every concept arises through the setting equal of the unequal. Just as it is certain that one leaf is never wholly equal to another, so it is certain that the concept leaf is formed by arbitrarily discarding these individual differences.
Gist of Idea
Leaves are unequal, but we form the concept 'leaf' by discarding their individual differences
Source
Friedrich Nietzsche (On Truth and Lies in a Nonmoral Sense [1872]), quoted by John Richardson - Nietzsche's System 2.1.1 n28
Book Reference
Richardson,John: 'Nietzsche's System' [OUP 2002], p.88
A Reaction
Nietzsche adds an interesting aspect to psychological abstraction, of abstracting away the differences between things, which we might label as the (further) capacity for Equalisation. If two cars differ only in a blemish, we abstract away the blemish.