...what if we all have some form of brain damage in common, so that none of us notice some simple and obvious fact?

Aren't things "obvious" by virtue of being noticed (or noticeABLE) by nearly everyone? Not trying to be difficult, but just trying to wrap my head around the idea that we could, all of us, be suffering such a severe cognitive malfunction. (I am thinking, here, of the liar's paradox.) And trying to wrap my head around the idea that now we could sit here in front of our computers and say anything worthwhile about it.

But for the sake of playing the game: "There are no coincidences."