Full Idea
Statements of the form 'a is F' aren't invariably positive ('a is dead'), and nor are statements of the form 'a isn't F' ('a isn't blind') always negative.
Gist of Idea
'A is F' may not be positive ('is dead'), and 'A is not-F' may not be negative ('is not blind')
Source
Fraser MacBride (Truthmakers [2013], 2.1.4)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.13
A Reaction
The point is that the negation may be implicit in the predicate. There are many ways to affirm or deny something, other than by use of the standard syntax.