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Comment by Kevin_Flynn

17 days ago

> CAN NOT is horribly ambiguous in English which is why RFCs use MUST and MAY.

Is it ?

I thought 'can' denotes a possibility, while 'not' negatates it ?

4 comments

Kevin_Flynn

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jameshart  17 days ago

It gets used more often to express prohibition not impossibility (“you can’t be in here” spoken to someone who manifestly is for example). Also disbelief (“this can’t be happening” about something that is happening). It’s just not clear which sense people mean it in, so it can’t be used when you want to be sure you can’t be misunderstood.

For another example of ambiguity, consider the sentence “A triangle can not have any obtuse angles”.

Is this 1) true, because there exist triangles with only acute angles, or 2) false, because triangles can have obtuse angles?

  • Kevin_Flynn  17 days ago

    > “A triangle can not have any obtuse angles”.

    Did you mean, "A triangle can only have one obtuse angle ?"

    Or was it a trick question ?

    • jameshart  17 days ago

      Precisely. It seems reasonable to say that ‘a triangle can only have one obtuse angle’ - but in some strict senses it is incorrect - one obtuse angle is not the only thing a triangle can have; it can also have none.

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