Can "cannot have past participle" express "inability" ?

Question

Can "cannot have past participle" express "inability" ?I learned that it's only used to express "impossibility", but I think it can express "inability" as well.For example,1. She/I can't have left yet because of the rain.or2. She/I have not been able to leave yet because of the rain.I think 1 expresses the inability of her or me, so 1 seems able to mean 2 and "can't have past participle" seems able to be used to express "inability".Is my thinking correct? and could you give me some more examples where "cannot have past participle" is used in this way?

CalifJim · Accepted Answer

fire1Can "cannot have past participle" express "inability" ?
I learned that it's only used to express "impossibility", but I think it can express "inability" as well.

Students tend to take these little word-tags like "impossibility" and "inability" much too seriously. They are usually no more than approximate hints about the usage of words.

Native speakers are relatively indifferent to fine distinctions like "impossible" vs "unable" when it comes to the use of the modal 'can' and its various forms.

Still, we might be able to narrow it down in the cases you are considering.

But let's start with 'must', and we'll see how 'cannot have' comes into it later. If I say The lights are on, so Richard must be home, I am saying that according to the evidence that the lights are on, the only reasonable conclusion I can arrive at is that Richard is home. This is a logical deduction.

Now the opposite of must is cannot (can't). If I say The lights are off, so Richard can't be home, I am saying that according to the evidence that the lights are not on, the only reasonable conclusion I can arrive at is that Richard is not home. This is a logical deduction.

Our next step is to show how both the affirmative and the negative forms can be placed in the past. These are must have and cannot (can't) have. That gives us these:

a) The lights were on, so Richard must have been home.
b) The lights were off, so Richard can't have been home.

It's the same logic, but about the past, namely,
a) I noticed that the lights were on, so I deduced by logic that Richard was home. I felt that the only reasonable conclusion was that Richard was home.
or b) I noticed that the lights were off, so I deduced by logic that Richard was not home. I felt that the only reasonable conclusion was that Richard was not home.

When it comes to the sentence you are concerned with in your post, it's saying It's raining, so she can't have left yet. The evidence you are using to support your claim is that it is raining. You connect this with the idea that if it's raining she won't leave. From this you draw the conclusion that she hasn't left yet. If she could go out at all, she could go out in the rain. There weren't any chains on her to prevent it. So in the case of the first example, this is a matter of logical impossibility rather than physical inability, which is what you wanted to know.

Consider that using "I" is such a statement is anomalous. Except in unusual circumstances, it makes no sense to make logical deductions based on your own behavior. It's raining, so I can't have left yet is about as strange as I'm still here, so I can't have left yet.

In the second sentence you specifically refer to ability:

She has (or I have) not been able to leave yet because of the rain.

This is different, and it's not the meaning of your first example. (By the say, in this case you can use 'she' or 'I', and it won't be anomalous.) The underlying situation, unstated, is that the rain is somehow a physical impediment to leaving. Perhaps there's a very heavy rain with strong winds that would make it dangerous to leave at that time. Here, even though you say "not ... able", you can take it as inability or impossibility. It doesn't matter how you characterize it in your description.

CJ

Can "cannot have past participle" express "inability" ?

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