In my native language, double negations "cancel out". I have been trying to learn English using movies and such things, but double negation confuses me. I see instances when double negation cancels out, and some when they do not.
For example, "I haven't got no money" is interpreted as "I have no money." "There wasn't anyone who didn't enjoy the party" is interpreted as "Everyone enjoyed the party."
Is this context dependent? If so, how does one decide which interpretation is correct?
In formal use, negation is "mathematical": double negations cancel out, as in your second example.
In colloquial use, however, double negation is usually "intensive": doubling a negation reinforces the negative sense.
Attention to context and emphasis will usually make it pretty clear what is intended.
On top of what others mentioned, I think a special type of double negation should be mentioned: Litotes.
Litotes is a stylistic figure of speech of creating a strong expression by phrasing it as a negation of its antonym; one normal negation combined with one inherent, a word containing the reversal or negative within its broader meaning:
Not bad!
Not an ordinary person.
It's not unlike...
Not too shabby
Not unheard of...
He's no saint...
Logically speaking, two negatives make a positive. And there are countless examples where this is true in standard or formal English as well:
I don't disagree. -> I agree.
The use of double negative resulting in a negative was not uncommon in Shakespeare's time
I never was nor never will be. (Richard III)
but was considered wrong after the 17th century - and is largely so still today.
But there is always the colloquial use of a language, which blatantly disregards grammar rules. These "mistakes" sometimes get really famous:
I can't get no satisfaction (The Rolling Stones)
Speaking of colloquial or non-standard use, how can we distinguish between double-negative-is-positive and double-negative-is-negative? The secret is the verb.
There are two "correct" ways to negate a statement - either via the verb or the object:
- I haven't got (any) money.
- I have got no money.
If there is a negation before and after the verb, it serves as emphasis, not as "canceling out".
In Standard English this does not apply - here the meaning "I have money." remains.
In the standard grammar, two negatives cancel each other. In some dialects, the double negation is used as emphasis.
The New Oxford American Dictionary has the following note about the double negative:
According to standard English grammar, a double negative used to express a single negative, such as "I don't know nothing" (rather than "I don't know anything"), is incorrect. The rules dictate that the two negative elements cancel each other out to give an affirmative statement, so that, logically, "I don't know nothing" means "I know something." In practice, this sort of double negative is widespread in dialect and nonstandard usage and rarely causes confusion about the intended meaning. Double negatives are standard in other languages such as Spanish and Polish, and they have not always been unacceptable in English. They were normal in Old English and Middle English and did not come to be frowned upon until some time after the 16th century. The double negative can be used in speech or in written dialogue for emphasis or other rhetorical effects. Such constructions as "has not gone unnoticed" or "not wholly unpersuasive" may be useful for making a point through understatement, but the double negative should be used judiciously because it may cause confusion or annoy the reader.
In English (and particularly British English), Double-negatives tend to be used for understated affirmation. For example, presented the question "How are you today", an American might answer
I am well, thank you
Whereas an Englishman would more likely answer
Not too bad, thank you.
Historically this is a style developed by the Puritans in the 16th and 17th Century, because the double negative reduced the amount of "emotion" in the sentence. This influence still carries in the UK, Australia and South Africa, but is largely absent from other Commonwealth countries and America who will more normally answer directly in the affirmative.
Additionally, in Standard English, it is possible to use double negatives to provide backhanded compliments:
Mr Jones is not an incompetent man.
Mrs Smith is not an unpleasant woman.
Note that English is not negatively concordant; that is to say that in English double negatives do not normally "emphasise" the negative-ness of the sentence.
I haven't ever been arrested
Is a negative sentence in its own right.
I haven't ever not been arrested
Is an avoided affirmation that the speaker has been arrested, rather than an emphatic denial or affirmation.
Finally, it's worth pointing out that there are some dialects of English - in particular in the African American Vernacular and Southern American Vernacular, English can be negatively concordant. Hence,
I've never had no money
Would normally never be heard in standard (Queen's) English, but may be said (in particular by African Americans or Americans in the deep south) to mean "I have never had any money".
Additionally, because negatively concordant sentences are "wrong" in the Standard English model, double negatives are sometimes employed to comic effect or to imply illiteracy, for example in the sentence:
We don't need no education.
He is never unavailable
He is always available
If you want to look at this from the perspective of whether a doctor is available at a certain time, both of these sentences mean the same thing. And the first one feels rather easy to understand.
However, when you use the word "not" twice in a sentence, many English speakers will find the sentence to be challenging. Such questions are easy to say incorrectly. You can use it correctly, and there is nothing technically wrong (or "rule-violating") with the grammar.
He is not unhappy
He is happy
These phrases may be slightly different. Although the prefix "un" usually means "not", this is a bit of a special case. The word "unhappy" usually means "sad". So, "not unhappy" mean simply mean that a person is "not sad", but could mean that the person is ambivalent/neutral, and is neither happy nor sad.
There are cases where it is appropriate to use a phrase like "not unhappy". Because this feels a little bit less straight-forward, this sort of phrase may require a little bit more effort for people to process in their brains. Speakers may assist by slowing down their speech, and perhaps speaking a bit louder, to help emphasize the individual words "not unhappy", and pause for a second or two so that the receiver can understand the full implications.
Here is another example:
He is not unavailable
He is available
(When speaking, the syllable "un" might be expressed with extra strength/emphasis.)
If this is advice given to a woman who wants a specific man to be a boyfriend, these might be different. The second sentence suggests that the man has no girlfriend. He is available for such a relationship. The first sentence might describe a man who isn't married (because he would be unavailable if he was married), but who is in a relationship that he's not happy with. An ambitious woman might try to convince him to end the unhappy relationship. So, he's not easily available. In fact, making him available might require a bit of effort. But, he's a lot closer to being available than someone who is happily married.
Note that this example is almost the same as the first example, when I said the sentences did mean the same thing. In this case, the sentences do have a small difference. When people talk like this and there is a difference, the difference is often small. In some cases, there isn't even any difference. In many cases, a person will need to evaluate a situation's possibilities to think about what is being said.
Sometimes different people will interpret things differently.
Example case: I am really into computers, and as an experienced computer programmer who has worked with bit-wise logic, I can easily understand the flipping of meaning caused by logical negation. I find myself to be more prone to correctly use double-negatives than many other people. On rare occasion, I even find a triple negative to be the most precise way to say something. Such a feat usually requires a bit more thought, as I make sure I speak with accuracy and technical precision.
However, my father is a fairly intelligent man who has demonstrated a particular weakness at being able to understand double negatives. When I use them, he says that he doesn't follow, or asks for a re-phrase.
Remember that communication is pointless if the receiver cannot understand what is transmitted. (This is true for both natural speech and electronic communication.) A lot of the other answers have focused on the grammatical rules. I'm pitching in, by adding this important perspective: a lot of times that double-negatives are used, it will cause significant strain for many audiences. This may be entirely appropriate for some scenarios, such as technical specifications which must be interpreted with precision (and is likely to be processed by intellectuals who may work with specific technical concepts, like logical negation, and thinking about which set elements are members of specific sets). For many other scenarios, such technical speech is often more cumbersome than useful. Talking this way is not what many people prefer for casual speech that is easily understood.
Most people can handle some simple double negation, with a bit of challenge. In some cases it is appropriate. In many cases, you may be better off seeing if you can re-phrase the sentence while containing the exact same meaning. In some cases, you can, and doing so will be easier.
One suggestion: Do not attempt to learn English from American movies as those tend to have a lot of slang in them!
Even the classic sentence someone mentioned "We don't need no education" is grammatically incorrect in its context. Double negatives cancel out each other and the sentence essentially means "We need an education"!
Similarly, never use double negation such as "I haven't got no money." This really means that "I have money."!
Notice that the people who generally speak such sentences in American movies are those who are marginalised, most often without an education and/or proper and systematic knowledge of English.
I hope I have answered your question on context and interpretation.
