illogical ineffective impossible irregular

I'd like to know if you agree that all these prefixes can always be replaced by a construction with "not" -- so "not logical" etc while having the same meaning.

And does "un" differ from the above prefixes in that it often yields a stronger meaning than "not", e.g. not happy can mean "neutral" and "unhappy" means "sad"

OR This person is unbelievable. (amazing, astonishing) This person is not believable. (not trustworthy)

  • 1
    For the first question, Not always. Being indifferent to something certainly doesn't mean not different = same. I'm sure there will be many more examples like that. And for un-, consider uncanny. Jan 11 at 11:58
  • The literal meaning of 'unbelievable' (and of 'not believable') is 'too strange to be readily believed'. Words like 'unbelievable, incredible, terrific' and so on are colloquial ways of saying 'very good'. Jan 11 at 16:43
  • Untied, unpacked, etc, can mean formerly tied, packed, etc, but not any more, with verbs untie, unpack, while not tied, not packed don't have that meaning. This seems more like a word puzzle than a practical question though.
    – Stuart F
    Jan 12 at 15:26

1 Answer 1


You are correct for the examples that you listed in your question. However, there are many exceptions where a word has a different meaning from what you might expect, and some examples are in the comments. I would add dis to the list of prefixes for not, for example, dissatisfied (not satisfied). But disinterested means impartial, not uninterested.

I don't think you can say "always" when it comes to English grammar rules because, regardless of the rule, there's probably an exception!

  • 1
    Your "disinterested" vs "uninterested" case is really interesting. Technically the OP's "not" substitution works for both of them, but each word operates on a different definition of "interested." It's a great case for keeping the pattern in mind as a guide, but treating each word as separate and looking it up in a dictionary.
    – YonKuma
    Jan 11 at 14:57

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