A professor was discussing how grammar varied across the world's languages:
Linguistically, many languages use a repeated positive for emphasis, e.g. "fast fast" to mean "very fast". This is a universal in creole languages.
Most languages use a repeated negative for emphasis or as a form of grammatical agreement. The idea that a double negative equals a positive does not arise from natural languages, it arises from a study of logic, and was then forcibly applied inappropriately to language. Because of this reinterpretation, in e.g. the prestige dialect of English, one must avoid double negatives, but in other dialects they arise naturally. For instance, "There ain't no more milk" simply means there's no milk, it doesn't contradict itself or turn into a positive "there is milk".
Arguments to the contrary invariably are not about linguistics, they're about logic or prestigious stylistics, etc.
The following discussion refers to the original version of the above which claimed that no language had a double positive. Participants may wish to edit their comments in this new light.
There is a rhetorical practice in Hebrew for example, of expressing the same idea multiple times in succession. I don't remember the name for it, but I believe C.S. Lewis refers to it in his Reflections on the Psalms and indicates that it is one of the few poetic forms which has the characteristic that it inherently survives translation. It seems to me that it is also an example of a double positive, or at least a multiple affirmation. YeahYeah! -- RaySchneider
double negative does not refer to multiple, re-affirming, negatives. Ergo multiple, re-affirming positives would not form a double positive.
Except that in the case of negatives, each successive negative flips the meaning. Multiple positives re-affirming each other is just the nature of a positive.
True, true.
good point.... however, all instances given here are repetitive: "yeah,yeah" is used in much the same context and usage as "no,no" would be. Which is very different from a double negative.
Yeah, right.
Did I miss something or aren't double negative precisely multiple, re-affirming, negatives? Remember to distinguish the meaning of a double negative in colloquial usage from that assigned by some grammar pedant.
"I don't know nothing." = "I don't know anything."
I have heard it argued that "anything" in this context is a negative. You couldn't say "I know anything". [On the contrary, I know anything I've learnt recently.]
I think you will find it hard to hear real life examples of double negatives meaning a positive. Even a grammar pedant knows that many people will misunderstand this usage.
In Julius Caesar, Mark Anthony, in his eulogy to Brutus, says repeatedly as a refrain, "Brutus is an honorable man". By the end of the monologue, it's clear that Anthony is saying that he is not honorable, but in fact a "brutish beast". Not exactly a double positive, but a similar literary form as in Psalms.
See also DoublePlusUnGood