Degrees of certainty for mustn't/cannot/may not/might not

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Tenacious Learner

Senior Member
Hi teachers,
I know that the percentages in modals are not black and white, but from books and surfing on the net I kind of like them? That's for the students.
How about these percentages? Do you agree with them?
100% (Be not) Robert isn’t sick.
95% (Must not) Robert mustn’t be sick.
95% (cannot) Robert cannot be sick.
Less than 50% (May not) Robert may not be sick.
Less than 50% (Might not) Robert might not be sick.

Senior Member
It doesn't quite work like that. Your examples of must and can aren't on the same scale at all, as they have quite different meanings (= I forbid Robert to be sick - it would be a disaster if he were).

If you add to the third example: Robert cannot possibly be sick, then it becomes 110%. Or not. After all, language isn't mathematics, and the English language is further from maths than most. Remember the scene in Dead Poets' Society when the teacher satirises this approach to language, and tells the students to tear that page out of their books?

owlman5

Senior Member
I'm not so sure about the first three, TL. As a teaching model, your range of probability looks okay to me. Make sure your students know that those numbers don't represent absolute truth. A speaker could reasonably use "can't" and "isn't" interchangeably to express his certainty (100%) that Robert is not sick: Robert can't be/isn't sick. I just saw him a minute ago and he was fine.

If spoken with a little emphasis, "must not" can also express certainty: (Looking out the window) That boy must not be sick. He's playing with his little friends right now, and he looks perfectly healthy to me.

Cross-posted with Keith, who disagrees a little.

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Tenacious Learner

Senior Member
Thank you to both for your help. That's why I've asked; I wasn't sure at all that the percentages really match the verbs.

TL

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