Polite way to refuse to answer a question
It sometimes happens that I am asked a question which I am uncomfortable answering for a variety of reasons (it invades my privacy, the answer may hurt the person asking, it is painful for me to discuss, it would violate a confidence etc.). I may also not feel comfortable explaining why I don't want to answer.
I remember a colleague of mine introducing me to the Far Eastern concept of Mu (sometimes translated as "unask the question"), which seems to quite aptly capture what I'm looking for.
What is the most polite way of expressing this in English?
Assuming you're willing to be clear that you don't wish to answer, I think the best response is probably "I'd prefer not to answer that". Anyone who asked you to explain why would in my opinion be incredibly rude; you'd be under no obligation to continue being polite if that happened.
Strictly speaking I agree with @Joseph's point below; the "most polite" form is probably "I'd rather not answer". This question reflects my belief that rather is more associated with casual speech and informality than prefer. So arguably it's more non-aggressive/acquiescent/polite. But I'd go for prefer - it's "firmer" to me, while still being polite.
If you don't even want to be so forthcoming as to actually admit that you don't want to answer, just ignore the question and start talking about something else. Sometimes people genuinely don't recognise what you mean by this, so they may ask you again. But if you repeat this "evasive tactic" and they ask a second time, you can safely assume they're being rude - so again, you're under no obligation to continue being polite. Depending on context, either tell them to "F**k off", or say something explicitly firm like "I'm sorry, but I think I've made it clear I don't want to answer that".
I'm not a Buddhist, but I think the answer "Mu" means something different than you intend. The answer "Mu" is along the lines of "is Schrodinger's cat dead or alive." "Mu" in that sense means "the answer is both yes and no". Not in the sense of "I don't know" but in the sense of "it is actually both at the same time."
However, the answer to your actual question is different. Your question is more about social graces than specific answers. Lets say for arguments' sake the question is the Bill Clinton classic: "Did you have sex with that woman?" There are a couple of approaches worth considering:
Change the subject, "thanks for asking, but I think you should really be asking about my welfare reform proposals..."
Go meta, which is to say, discuss the discussion rather than the contents: "that depends of the what the meaning of 'is' is."
Reply with a question: "Did you have sex with your wife last night?"
Cite other people's concerns, rather than your own: "I'd be happy to discuss that with you, but I don't feel it would be fair to Monica."
Go oblique: "We had cigars and brandy"
I imagine a few other ideas might be appropriate too.
If you want to be forthright, you can say, "I'd rather not say." If you want to be a little less blunt, you can say, "It's hard to say," which as ambiguous enough to allow a little freedom. In a more informal setting, "You never know..." is a good way to deflect a question.
My favorite is "One finds it difficult to say." My second favorite is along the lines of
"You might very well think that. However, I couldn't possibly comment,"
per Francis Urquhart. That might not yield just the effect you want, but see below.
The phrase easiest to say and stick to is "No comment"; practice saying it and meaning it, and after saying it provide no further comment. An alternative is verbiage. One could develop a paragraph or even a sermon around "I couldn't possibly comment," with a few highly boring sentences mixed in as well. This approach is rather more polite than confronting the other party with facts they aren't comfortable with, and in any case that approach can lead to acrimony, which is never polite.
If you anticipate the problem beforehand, think of some topics to use for changing the subject when uncomfortable with the current one. Changing the subject can be subtle or unsubtle; it usually isn't regarded as impolite, although may be thought evasive.
You can respond with another question. A good strategy that is often appropriate is to shame them with, "Why would you need to know something like that?
Or you can try the bureaucratic approach: "It's my/our policy not to respond to that type of question". This also works well when politely rebuffing telemarketers: "I'm sorry, we have a policy of not responding to telemarketing calls".