While our conscious minds understand negation the subconscious does not. Sounds unreal, right?
Back to the election:
Attack adds ran over and over "Justin Trudeau, he's just not ready!"
The conscious mind hears the statement as intended, the subconscious hears:
"Justin Trudeau, he's just ready!"
And here is the kicker for the writers of the attack ads - the real driver of all human action is the subconscious. It is our more primitive animal brains that drive our behaviours.
So the lesson is:
After every first draft of a piece you must read it again omitting the negation to understand what message will go into the subconscious of your reader.
Know the message you are really writing, be clear!