Let's take this sentence as an example

He is able to move.

Now, what is the best negation of that action between those two?

  • He is not able to move.
  • He is unable to move.

And what makes one better or more correct than the other?

Edit: In case these sentences aren't good enough, give examples of your own if you please.

  • 1
    In that particular case, either is acceptable, and (in my view) neither is better than the other - at least in the absence of any other context.
    – TrevorD
    Commented Oct 13, 2013 at 15:46
  • In response to your Edit, I don't know what you mean. It's YOU asking the question - what are you asking for examples of?
    – TrevorD
    Commented Oct 13, 2013 at 16:00
  • possible duplicate of "That my results are not reproducible" or "that my results are unreproducible"? Commented Oct 13, 2013 at 16:03
  • @TrevorD, you said - at least in the absence of any other context. –, so if you have in mind any example where one is better than the other please feel free to give it. Commented Oct 13, 2013 at 16:05
  • No, I don't have any in mind. Actually I meant in the context of what precedes or follows the sentence you wrote. Although I have nothing in mind, I can conceive that there might be instances where one might be preferred.
    – TrevorD
    Commented Oct 13, 2013 at 16:25

2 Answers 2


In the Corpus of Contemporary American English, unable to is nearly eight times more frequent than not able to. The difference is even greater in the British National Corpus, where unable to is more than thirteen times more frequent.

You can draw your own conclusion from those numbers, although there may be contexts in which not able to is preferable. In practice neither is likely to be used as much as can’t, which is vastly more frequent in both corpora. For example, in a dialogue in which one speaker asks ‘Why doesn’t he get up?’ another speaker would almost certainly reply ‘He can’t move’, rather than either of the two alternatives.

  • The latin "alter" always refers to two, and only two possibilities ; I feel uncomfortable with "two" alternatives, four choices then. The meaning "a choice between many others" is widespread, such as in "the alternative medicines", but is it not still a mistake ? Commented Oct 13, 2013 at 19:29
  • I know what Latin alter means, but that is not necessarily any guide to what the English word means. The OED’s entry for alternative includes several definitions which mean two or more. Commented Oct 13, 2013 at 19:48
  • 1
    @MarkThorin: that is called the etymological fallacy. The meaning of a word (or phrase) lies in what it is used and understood to mean, and nowhere else.
    – Colin Fine
    Commented Oct 13, 2013 at 22:09

The meaning is strictly the same ; when you have the negative form of an adjective ("-in", "-un", "-il", ...), and sure (not "inable" or "unaccurate" for instance !), it is always better to keep the affirmative form of the sentence.

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