Possible Duplicate:
Is there a rule about double negations that aren't meant as double negations (e.g. “We don't need no education”)?
Meaning of “you don't need no memory” and its grammar or rhetoric
Is there any consensus yet on how double negations should be treated?
For instance:
I don't need no doctor. (coll.)
Does the speaker need a doctor or not? Some authorities emphasise that logic prevails, in which case the speaker would need a doctor (no and no means yes). However, there are plenty of others who say this is not true.
When one starts from the meaning of what a speaker wants to express, he would easily come to tell that double negations do NOT make a positive sentence. The aforementioned logic-thinker, however, would just call that rum ram ruf and continue by saying that the world needs to learn their language again.
So: intention of speaker > logical structure of language, or not?
Also, as a side note, my mother dialect (a Dutch (Flemish) dialect) does have double negations as its default way of making a sentence negative.
Da'k 'et nie-j-en wee = That I not-en know