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It's pretty common, especially in video games' mission objectives, to state:

A person X must not die.

Would this be rapidly different if constructed like this?

A person X cannot die.

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Rapidly means 'quickly'. Is that what you meant? – Karl Apr 6 '11 at 12:04
up vote 10 down vote accepted

Yes a big difference.

Cannot die means the person is immortal and incapable of death.

Must not die means that they are mortal, thus very capable of death and the mission requires you to keep them alive.

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oh, this didn't occur to me at all, haha. if at least another person can agree with this by giving it a +1 and nobody has anything more to add, I'll accept this answer :) – RiMMER Apr 6 '11 at 10:23
Cannot could be used in the form "Person x cannot be allowed to die" – Robb Apr 6 '11 at 10:32
As Mitch's answer shows, "cannot" is ambiguous here. – Colin Fine Apr 6 '11 at 14:18

In this setting they are pretty much the same. Language can have a literal meaning, a canonical meaning (the first thing you think of), a metaphorical meaning ("the face of a cliff"), and implied meanings.

Literally, "cannot" means logically or physically impossible: "we cannot pick up that huge boulder". And "must not" means not allowed. But both here are said in order to convey that, loosely, you really don't want to let person X die and you should try tomake that not happen. So they're pretty similar here.

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That being said, if you happen to ever write dialog for mission objectives, the clear winner here is "must not" because it is completely unambiguous. – MrHen Apr 6 '11 at 14:58

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