Can't be VS Mustn't be

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Opala

New Member
English - US
Which sentence is more correct and is there a difference in meaning.

1. You can't be right
2. You must not be right.

Are these two interchangeable?
 
  • peptidoglycan

    Senior Member
    Turkish
    1. You can't be right.: It is not possible that you are right.
    2. You must not be right.: There is enough evidence that you are not right. (Logical conclusion)
     

    Wordnip

    Senior Member
    British English
    'You must not be right.' suggests that it is forbidden that you be right. This would require a very unusual context for it to make sense, but it is possible: it could be an instruction to someone who is to act a role in which they must not say something which is correct or right. "You must tell a lie, you must not be right under any circumstances."
     

    Opala

    New Member
    English - US
    I have a debate with another teacher who claims that they are interchangeable and actually mean the same.I personally have never used the must not version and it sounds awkward and even unnatural to me.
     

    sound shift

    Senior Member
    English - England
    I know for a fact (from several WR threads) that there are people who say, "He must not have arrived yet" as a logical conclusion. I could not say that; I would use "He can't have arrived yet." Consequently I wonder if there are any people who could say "You must not be right"; I don't exclude it (yet).
     

    Wordnip

    Senior Member
    British English
    I disagree with peptidoglycan who writes, "You must not be right.: There is enough evidence that you are not right. (Logical conclusion)"., because the correct conclusion is that you cannot be right. Must not, as I said above, means it is forbidden to be right; a moral stricture. We don't usually say that. The two sentences are not equal.
     

    JustKate

    Senior Member
    Maybe this is just me, but I'd probably use "You must not be right" only if my intended meaning was "Steps have to be taken to make sure that you are not right." This isn't something I'd say very often, obviously, but must is, in AmE, a pretty emphatic word, so we tend to use it only when we want to be very emphatic and very definite.

    Edit: While we use both must and must not, we almost never use mustn't, by the way.
     

    peptidoglycan

    Senior Member
    Turkish
    Betty Azar's Understanding and Using English Grammar book says:

    100% sure: Sam isn't hungry. (The speaker is sure that Sam is not hungry.)
    99% sure: Sam can’t/couldn’t be hungry. (The speaker believes that there is no possibility that Sam is hungry (but the speaker is not 100% sure). When used in the negative to show degree of certainty, couldn't and can't forcefully express the idea that the speaker believes something is impossible.)
    95% sure: Sam must not be hungry. (The speaker is expressing a logical conclusion, a "best guess.")
    less than 50% sure: Sam may/might not be hungry. (The speaker uses may not/might not to mention a possibility.)

    Betty Azar, as you may know, is from US.
     

    JustKate

    Senior Member
    Well, as usual, context is everything. I agree that in "Sam must not be hungry," must indicates a logical conclusion (at least usually). However, that's not what it means - at least not to me - in "You must not be right." If my meaning was "The logical conclusion is that it's not possible that you're right," I'd use can't.
     
    Last edited:

    Wordnip

    Senior Member
    British English
    Well, as usual, context is everything. I agree that in "Sam must not be hungry," must indicates a logical conclusion (at least usually). However, that's not what it means - at least not to me - in "You must not be right." If my meaning was "The logical conclusion is that it's not possible that you're right," I'd use can't.
    The logical conclusion would probably be 'Sam is not hungry' (depending upon the argument). 'Is not' is not equal to 'must not'.
     

    JustKate

    Senior Member
    I agree that is not is not equal to must not. But if you're saying that must not isn't used to indicate a logical conclusion, I have to disagree because in AmE, it sometimes is. That's not what it's always used for - it isn't in "You must not be right." But it is used that way fairly often.
     

    Wordnip

    Senior Member
    British English
    I agree that is not is not equal to must not. But if you're saying that must not isn't used to indicate a logical conclusion, I have to disagree because in AmE, it sometimes is. That's not what it's always used for - it isn't in "You must not be right." But it is used that way fairly often.
    'Sam must not be hungry' can be the conclusion of a logical argument, yes. I am asserting that it cannot simply be substituted for '... is not ...' in a conclusion (and vice versa). Logic is a precise discipline and you would have to advance further argument based upon the premises to show the equivalence (very unlikely that this would be possible. This is one reason why 'correct' vocabulary/grammar is important).

    Related to this is the 'Is-ought' problem. You might find this article of interest:

    http://en.wikipedia.org/wiki/Is–ought_problem
     
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