mu

A monk asked Joshu, "Does a dog have the Buddha nature?" Joshu retorted, "Mu!"

This word comes from Chinese and has the meaning (in Chinese) of "nothing". It's used in the zen koan above to have a slightly different meaning, something along the lines of "Neither yes nor no, since the question is built on incorrect assumptions." The word is a non-word, with a meaning that cannot be expressed in the language that the question was asked in, and, presumably, any language. The language it could be answered in, alternatively, would not be able to ask the question. The statement "0 equals 1" is a non-theorem in number theory, since you cannot prove that the statement is correct using the assumed axioms of the system. In other words, that statement has no meaning in number theory.

If someone asked you "Have you stopped beating your wife yet?" and you did not have a wife, could you answer yes? You cannot stop something you have never done, nor can you say you have not stopped as that implies that you are continuing to beat her. You can answer, now, "mu" or "0 equals 1", I guess.

When I was a kid I had a lot of fun asking my sister, "Are you PT?" If she answered no, I would say, "You're not potty trained?" If she answered yes, I would say, "You're part-tiger?" Or something like that, I forget the answer to yes exactly. In any case, after a while my sister would have to concede to being part tiger in order to avoid agreeing that she's not potty trained. I didn't realize it, but I was close to enlightenment even then! Now, having realized the system's tricks a bit more, of course I'm much further away.