My interpretation of Plato’s doctrine of negative assertions (or “not-being) in the Sophist at (roughly) 257e
Plato’s doctrine of negative assertions:
Stranger: May we not say that the existence of the not-beautiful is constituted by its being marked off from a single definite kind among existing things and again set in contrast with something that exists?
Stranger: So it appears that the not-beautiful is an instance of something that exists being set in contrast to something that exists.
Stranger: What then? On this showing has the not-beautiful any less claim than the beautiful to be a thing that exists?
Theaetetus: Just as much
Stranger: And we must also put the not-just on the same footing as the just with respect to the fact that the one exists no less than the other.
The existence of the not-A (the form not-A?) is constituted
(a) by its being separated from a single (definite) Form among existing things (particulars)
(b) by its being separated from something that exists (form or particular??)
If and only if not-A then (p) A is distinguishable from a single definite form F among existing F-particulars and (q) A is distinguishable from other non-F particulars and/or non-F forms