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...Between:

I don't know that it isn't all true
and
I don't know if it isn't all true??

There are little differences between the two I believe.

The first means: He doesn't know that it isn't all true; therefore may believe the contrary.

The second means: He doesn't know if it is true or the contrary.

Overall, both can be used to convey the same idea of doubt.

Right???
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Hi wholegrain

This is what those sentences mean/suggest to me:

I don't know that it isn't all true. => I think it is all true, and there is nothing I know that would lead me to believe it isn't.
(This sentence sounds natural to me.)

I don't know if it isn't all true. => Someone (else) said it isn't all true, and I'm not sure whether that is right or not.
(I'd feel more comfortable with additional context for this sentence.)
Wait, the first is a double negative and the other is not?

Btw, there is no context.
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The first one has two different shades of meaning to me.

The first is a way of suggesting one thing by denying the opposite. It's a bit like "I'm not unimpressed", for example. I expect there's a technical word for this figure of speech, but I can't bring it to mind right now. Anyway, the net effect is that you're saying "I rather think that it's all true" (not so positive, then, as "I know that it's all true").

In another sense, it just means that you don't know. It might, depending on context, mean that you think it isn't all true (but don't know), or it may indicate a neutral standpoint.

A: How do you know that it isn't all true?
B: I don't know that it isn't all true.


(The presence or absence of the word "that" makes no difference for me.)
You can say:

I don't know that it isn't all true, or but, or but that, or but what, it is all true.

So why do "I don't know that it isn't all true" "I don't know but it is all true" "I don't know but that it is all true" and "I don't know but what it is all true" all mean the same thing???
Hi, Wholegrain,

I just looked to see where "but" sneaked into the thread and I couldn't find it.

This is very nostalgic for me. My grandfather was a yankee farmer, and he used to say that. I haven't heard it since he died, circa 1960. When he said it, it seemed like a contraction - I dunno b'what it's all true.

- A.
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wholegrainYou can say:

I don't know that it isn't all true, or but, or but that, or but what, it is all true.

So why do "I don't know that it isn't all true" "I don't know but it is all true" "I don't know but that it is all true" and "I don't know but what it is all true" all mean the same thing???

The variants with "but" are all obsolete in mainstream English (though possibly they're still used in some dialect somewhere). I don't know anyone who would use any of them. It's the sort of thing you'd read in old books.

"but" has (or had) a sense of "otherwise/except" here. So, "I don't know but that it is all true" means "I don't know otherwise than that it is all true". In other words, "I believe (or tend to believe) that it is all true".

"I don't know but it is all true" is the same, just with the omission of "that" (as is also common in modern English; c.f. "I don't know that it isn't true"/"I don't know it isn't true"). "what" here is just an obsolete/dialect/"non-standard" word for "that". In Britain, at least, you do hear some people say "what" for "that" (for example, "It's the one what I saw in the shop"), but it really jars with me.
I don't know but I think, but here meant but that as in that, that's why I was not quite sure what it meant.
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