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"Must not" vs. "don't have to" is a famous false friends for many languages. That said, what about a phrase like, say, "nobody must know":

Does that mean that a) nobody is allowed to know, or b) that obviously nobody knows, or both? Is that sentence OK, poor style, or to be reworded to make unambiguous? How would I use "must not know", as in "is not allowed to know" together with "nobody"?

  • "Nobody must know that" = a) nobody is allowed to know. If you want to use "is not allowed to know", you can say "Nobody is allowed to know that". – Vilmar Dec 6 '13 at 11:26
  • This illustrates the phrase well, explainxkcd.com/wiki/index.php?title=515:_No_One_Must_Know – Damkerng T. Dec 6 '13 at 11:29
  • Just picture a more complete example: Nobody must know the answer, otherwise they would have responded). Wouldn't that be equally correct? – Ingmar Dec 6 '13 at 11:29
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    If I'm not confused enough between Ingmar & @Vilmar, both (a) & (b) are possible implications: (b) (It must be the case that) nobody knows. Guess the second is more colloquial than formal. – Kris Dec 6 '13 at 13:32
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    That's the epistemic must, which is rather rarer than the deontic must in the negative, perhaps because of the ambiguity. It's grammatical, though. – Colin Fine Dec 6 '13 at 14:25
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Both. Without further context, "nobody must know" is ambiguous.

Consider:

Nobody must know about my new job.

This is ambiguous. The speaker might be ashamed of his or her new job, and not want anyone to know. The speaker might also be remarking that everyone is simply unaware of his or her change in employment.

This could ruin us. Nobody must know.

In this case, the speaker is referring to a damaging piece of information that must be kept from everyone, except possibly the person or people to whom he or she is speaking.

We don't have any customers since we moved. Nobody must know our new location.

The speaker is remarking at the apparent lack of knowledge of the new location by the customer base.

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    Right. Like all modals, must has an abstract epistemic sense about probability, and a deontic epistemic sense about obligation. Must not S, in either sense, is logically ⃤ ¬S; i.e, Necessary (Not S) = Not-S is Necessary. Necessary can be either logically (This must not be the correct spell) or authoritatively (You must not open this door). – John Lawler Dec 6 '13 at 19:29

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