The half-baked philosophical conundrum that keeps me up at night most often is simply: why is my consciousness limited to me in particular? Why am I not everyone? Or everything? or nobody? Or you? Or an ant? Or an ant colony? How did I find myself in this particular stream and what is the nature of the boarders that keep me within it?
This idea generally frustrates and freaks me out and I doubt there is a sensible answer, I doubt it's productive to think about, but I worry on it over and over.
