I am trying to determine the correct meaning of a phrase: They can (but must not) be green.

The person who wrote the phrase insists that it means they can be green, but could be some other color.

To me, the phrase describes two mutually exclusive requirements.

What is the correct interpretation of the phrase?

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    The utterance They can (but must not) be green is semantically invalid (it doesn't make sense). The nearest valid utterance would be They can (but need not) be green. But idiomatically we'd normally express that as They can be green, but need not be. Or They can be green, but they don't have to be (where most native speakers would pronounce have as haff). Commented May 10 at 16:15
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    I’m voting to close this question because it's askipng about the meaning of an invalid utterance. Commented May 10 at 16:18
  • @FumbleFingers - surely 'can' might express possibility, rather than permission, so that although green is among the colours that are technically possible, the things under discussion are not be green? I agree that the utterance is so awkward that no native speaker would make it. Although green is in the range of possible colours, it is not to be chosen. But why would anyone say it? Commented May 10 at 18:29
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    @MichaelHarvey: I don't know which Gricean maxim is violated by juxtaposing can = theoretically possible by the laws of man with must not = disallowed by the laws of man, but I'm sure it does. Anyway, since it's only a theoretical possibility that never occurs in practice, it should be could, not can. Commented May 11 at 1:07

1 Answer 1


You are right.

I would guess that this has been written by a speaker of a language such as German, where "Ich muss nicht" means "I don't have to", not "I must not".

  • I once read a book about British psychological warfare in World War 2 (Sefton Delmer, etc) and, on the subject of propaganda leaflets, the author said that the team felt that the verb 'mussen' had a strange psychological power over 'the German mind' (this was 1942). An expression using it, once read, practically 'took on the power of an order'. A lot of wartime stuff written about the Germans by British people seems alarmingly prejudiced these days. Commented May 10 at 18:37

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