Kriemhild

If the cat box is not opened, then both the possibilities of a living cat and a dead cat will exist inside the box, from the observer's perspective.

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If you're referring to the sig quote, it's referring to events where nobody actually knows what really happens. So a fictional story that would spread about those events would eventually become established as the truth. Even though nobody actually knows what really happened, they would take the fictional story as the truth for those events.