Full Idea
So 'the not beautiful' turns out to be ..both marked off within one kind of those that are, and also set over against one of those that are, ..and the beautiful is no more a being than the not beautiful.
Gist of Idea
The not-beautiful is part of the beautiful, though opposed to it, and is just as real
Source
Plato (The Sophist [c.358 BCE], 257d)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.281
A Reaction
[dialogue eliminated] This is a highly significant passage, for two reasons. It suggests that the Form of the beautiful can have parts, and also that the negations of Forms are Forms themselves (both of which come as a surprise).