1

Here are sentences:

Tom walked into a wall. He can't have been looking where he was going.

What's the meaning of the second sentence above? Does it mean:

1) Tom can't look where he was going AFTER he walked into the wall.

2) Tom walked into a wall, BECAUSE he wasn't looking where he was going.

  • What does this question have to do with the word "must"? – DJMcMayhem May 9 '15 at 7:27
  • 1
    It can't be true that he was looking where he was going. (an inference the speaker thinks is very likely to be true) – snailcar May 9 '15 at 7:48
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Sometimes we use modal verbs to show that we are making a deduction. We show that we are using the evidence we have to decide whether something is definitely true, maybe true, or definitely not true.

Must

If we want to show that we think our evidence shows that something is definitely true we often use must:

  • This animal has feathers. It must be a bird of some kind.

In the example above the speaker is saying that their evidence tells them that the animal is definitely a bird.

Might

If we think evidence shows that something is possibly true, we sometimes use the verb might:

  • It's going to rain tomorrow, so we might get wet.

The speaker above is saying that it is possible that they will get wet.

Can't

When we want to show that something is definitely not true, we sometimes use can't:

  • That animal can't be a mammal, because it has feathers.

Here the speaker is saying that their evidence shows it is impossible that the animal is a mammal. Some speakers, might use must and a negative verb phrase:

  • That animal must [not be a mammal], because it has feathers.

In British English we do not like to use must this way. Most British English speakers will not find the sentence above grammatical. However, some speakers of American English will find the sentence perfectly fine.

Notice that if we want to say that our evidence tells us now that something was definitely not true in the past, we use can' and a perfect construction:

  • That animal can't have been a mammal, because it didn't have feathers.

This sentence is perfectly grammatical (excuse the pun!). We do not need to use could here.

The Original Poster's sentence

Tom walked into a wall. He can't have been looking where he was going.

This sentence means that the evidence shows that it is definitely not true that:

  • Tom was looking where he was going.

We could try to phrase this sentence using must. Some speakers will not find this good, although they will understand it:

Tom walked into a wall. He must not have been looking where he was going.

We can make it more acceptable if we don't negate must but negate one of the verb phrases:

Tom walked into a wall. He must have been not looking where he was going.

Note: There is an interesting question about can't and mustn't on our sister site, ELU.

Hope this is helpful!

-1

The first sentence is grammatically incorrect. "Can't" is a present tense verb, but the sentence is in past tense. It should instead read:

Tom walked into a wall. He couldn't have been looking where he was going.

Sentence #1 (Which should also have "couldn't" instead of "can't") means

Tom was walking, and then he walked into a wall. After walking into the wall, he was unable to look where he was going.

Maybe he hit his head really hard. Or poked his eye. Or something like that. Sentence #2 means:

Tom was walking and not looking where he was going. Because he was not looking where he was going, he walked into a wall.

The first sentence doesn't mean either of these. A better paraphrase for the first sentence is:

Tom walked into a wall. If he had been looking where he was going, he would not have walked into a wall. Therefore Tom was not looking where he was going.

  • But the first sentence is entirely grammatical! It means "he must have been not looking where he was going". – Araucaria - Not here any more. May 9 '15 at 10:35
  • @Araucaria I agree that that's the meaning of the sentence, but I don't think it's grammatical. I don't think you can use "can't" in a past tense like that. I think it should be "couldn't". – DJMcMayhem May 9 '15 at 14:04
  • That would be more logical perhaps. However, it's not how the grammar actually works :-) – Araucaria - Not here any more. May 9 '15 at 14:06
  • @Araucaria would you say that "he can have been looking" is grammatical? – DJMcMayhem May 9 '15 at 14:17
  • No, we don't use positive can for epistemic deductions, only can't. We might be able to use it in reply to someone who has used epistemic can't. So your example above could work as a reply A : He can't have been looking. B: He can have been looking. But it still isn't very elegant! – Araucaria - Not here any more. May 9 '15 at 14:24

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