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While reading a book, I encountered the following phrase:

...must not have (told)

I found that confusing because my understanding was that "must not" is used to indicate that something is strongly forbidden, but that doesn't seem to make sense here, Can anyone clarify?

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The modal verb "must" can convey either (1) necessity, as with your example of strong prohibition, or (2) probability. In this case it conveys high unlikelihood, i.e. "He did not know, so she must not have told him."

Check Cambridge Dictionary for more examples.

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Most modals (eg must, should, can, will, may) have two rather different meanings: a deontic meaning, about how the world is (including things like necessity and obligation), and an epistemic meaning, about the state of our knowledge.

As you say, the deontic meaning of must is "strong necessity or obligation". But the epistemic meaning is something like I am sure that.

In some contexts, either is possible, and there is a formal or actual ambiguity: "He could win the prize" can mean "He has the ability to win the prize" or "I don't know whether he will win the prize".

But with the past perfect "must have", usually only the epistemic reading makes sense: "he must not have X" usually means "I deduce that he certainly didn't do X", or "I'm sure he didn't do X".

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    Perhaps worth noting that the epistemic usage (expressing speaker's certainty, rather than subject's obligation) is very rare with contracted He mustn't be X, but perfectly natural with the near-synonymous He can't be X. In which context I notice that quite a few written instances of It mustn't be true! are juxtaposed with things like It can't be true! / It's not true! to disambiguate / underline the intended "emphatic disbelief" meaning. Sep 9, 2019 at 17:01

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