Oh if you could find it I would be super grateful. I've done my usual rounds such as searching on Google Books and found nothing. Made me start to wonder if it had been Jung instead.
I haven't read that before, but I am wondering if that is related to that specific phenomenon, or rather if it's disrespectful to refer to the king, and say "ass" in the same sentence. Do you remember?
Yes, the example was to illustrate that the unconscious doesn't process negation the way the conscious mind does. The fact that you said the king is not an ass proves that you had the idea that he is. So I think your alternate interpretation of the principle is not so different from the first. The claim is that negation is a logical/abstract construct that exists at a higher cognitive level not recognized by the (allegedly) more primitive unconscious, which deals in concrete language and images.
Another way to put it is that psychologically, a statement and its negation are not opposites but rather go together. The same idea comes up in hypnosis, where it is said that the unconscious mind drops the "not" and simply receives the images that are given. For the same reason, it's better to say "Remember" than "Don't forget", which is a kind of subliminal invitation to forget. And so on.
Edit: I remember reading this years ago and have been unable to track it down. Would love it if somebody did.