Apologies for posting so much in the June Open Threads. For some reason I'm getting many random ideas lately that don't merit a top-level post, but still lead to interesting discussions. Here's some more.
How to check that you aren't dreaming: make up a random number that's too large for you to factor in your head, factor it with a computer, then check the correctness by pen and paper. If the answer fits, now you know the computing hardware actually exists outside of you.
How to check that you aren't a brain in a vat: inflict some minor brain damage on yourself. If it influences your mind's workings as predicted by neurology, now you know your brain is physically here, not in a vat somewhere.
Of course, both those arguments fall apart if the deception equipment is "unusually clever" at deceiving you. In that case both questions are probably hopeless.
How to check that you aren't dreaming: make up a random number that's too large for you to factor in your head, factor it with a computer, then check the correctness by pen and paper. If the answer fits, now you know the computing hardware actually exists outside of you.
A similar method was used by Solaris protagonist to check if he isn't hallucinating.
This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.