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Is "can’t help but" considered to be a confused mix of the expressions "can but" and "can’t help"? If not, what is the difference between "can help but" and "can’t help but"?

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Are you asking what the origins of the phrase are? Nobody knows, or rather, it would take a lot of research to find them, and they would be only opinions at best. The difference between can help but and can't help but is that the second one contains the negative that triggers it, while the first one doesn't, which means that there has to be some other negative lurking around to license it, like She doesn't think I can help laughing at that. If it doesn't occur within the scope of a negative, it's ungrammatical. –  John Lawler Feb 23 at 19:03
    

5 Answers 5

I can’t help but think this is a difficult question means that I have no alternative to thinking that this is a difficult question. I can help but think this is a difficult question is not something a native speaker would say. The combination can but is used in sentences such as You can but try, encouraging the person addressed to attempt a task whose outcome is uncertain.

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A native speaker might say something like "I can help, but think this is a difficult question"; but that's an entirely different kettle of fish. –  Marthaª Sep 6 '12 at 20:56
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@Marthaª: They might, but I would expect them all to say something like "I can help, but I think this is a difficult question". –  Adam Robinson Sep 6 '12 at 21:07
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Or, "I can help, but it would be too much trouble so I won't." –  Jay Sep 6 '12 at 21:11
    
While I agree that "can help but" isn't common in modern English, it is quite common the further back you go. Although I am not sure if there is a very stringent different between the two terms, they both convey relatively the same meaning. There are numerous instances where I've come across this, especially in financial reports and metal illness documentation dating from the late to early 1800s. Don't ask. –  Tucker Feb 19 at 20:01
    
@Tucker: metal illness => zinc poisoning? Bullet wounds? Rashes from ill-fitting armour? =-) –  Mathieu K. Feb 20 at 4:10

Can help but Infinitive and can help Gerund are both Negative Polarity Items (NPIs).

That means that they can't occur grammatically outside the scope of some Negative trigger. So they're just fine in a Negative context (here with can't instead of can)

  • I can't help thinking he got the better of us in that deal.
  • I can't help but remember the smile on his face.

but they're terrible outside a negative environment (what a difference an -n't makes!)

  • *I can help thinking he got the better of us in that deal.
  • *I can help but remember the smile on his face.

    Summary: If Negation is involved, look for NPIs before doing anything else.

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This "negative polarity" business has quite a long reach! I'm assuming @Barrie's "You can but try" falls into the category specifically because it raises doubts over how likely it is that trying will lead to succeeding. –  FumbleFingers Sep 6 '12 at 20:18
    
That particular but is equivalent to only and is indeed negative. As are questions, comparative and superlative constructions, and "syntactic constructions (This is it, isn’t it? Not any big ones, he didn’t), variation (so didn’t I; ain’t got none), morphology (-n’t, -free, un-), (morpho)phonology (do/don’t), intonations (‘Riiight’), and lexemes sporting negation that is overt (never), incorporated (doubt, lack), calculated (few), entailed (prohibit), or presupposed (only)" –  John Lawler Sep 6 '12 at 21:25
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Perhaps people are missing that can’t but is one of those true double-negatives that makes a positive. –  tchrist Sep 6 '12 at 21:49
    
I can't help but think the first "but" in this sentence has essentially that same "only" sense. Actually, the more I think about "I can but do X", and "I cannot but do X", the more it seems to me they can mean exactly the same thing. I do recognise that idiomatically, the can version can imply I can do no more than X, and the cannot version can imply I must do X, but it still seems pretty odd that in some contexts the negation of "can" doesn't necessarily affect the meaning. –  FumbleFingers Sep 6 '12 at 21:50
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It's standard grammatical practice to mark ungrammatical sentences with asterisks. That's how we can cite them. –  John Lawler Feb 25 at 5:26

This usage comes from a very old meaning of "but" in which it means, literally, "outside". Its original form is the Anglo-Saxon "be outan", which we can gloss as "by out", i.e. "outside". This use is still found in Scots where they might say "He's waiting but the house".

Anyway, there's your etymology lesson. As far as i know, this ancient prepositional "but" is the ancestor of all the many uses of "but" that are in English today, and it has as its not too remote ancestor the "but" of

No one but me heard the remark

which without sounding too weird , we could put equivalently as

No one outside me heard the remark.

It looks to me like a lot of the NPI uses of "but" come from this old prepositional meaning. You use this sort of but to say that nothing "outside" a particular class has interesting property X. "You can't but try" means you have no other option "outside" of trying.

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"You can't but try" -> should be "You can but try" (as in the answer proposed by @BarrieEngland). Google it, for instance. The one with "can't" doesn't seem to exist. –  Mathieu K. Feb 20 at 4:09
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I don't think that was Barrie England's point, and I disagree with you. Googling, I found this: dictionary.reference.com/browse/can+not+but Can but try is also correct and perhaps more modern, but I believe this usage came out of gradual elision of the not in "can't but". This is where minutive buts come from, as in "Nay! We are but men", which means that we are not but men, i.e. nothing outside of men. –  mander Feb 20 at 6:06
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On va s'coucher moins niaiseux (I'll go to bed smarter tonight), as they say around here. –  Mathieu K. Feb 20 at 21:48

"Help" here is not used as in to assist.

"I can't help it."

This means I can't stop/control myself, or I have no other choice.

Some examples:

I can't help but laugh at her misfortune.

When the music gets funky, I can't help but dance.

I can't help but wonder if he was just being nice to get something for himself.

I can't help it! It's involuntary.

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Both have different meanings.

I can't help, but I know someone who can.

Meaning: I can't help, but I can refer you.

I can help, but I don't think it is possible.

Meaning: I will help you, even if it is impossible to do.

I can't help but think you are crazy.

Meaning: Your behavior compels me to think you are crazy.

No native speaker writes "I can help but .. " Because there should be a comma after help.

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