Some people say double negatives are illogical because of math. In math two minuses make a plus. We can’t use this however because three minuses would make a minus again and in language this isn’t like this. So double negatives can’t be looked upon as logical.
Double negatives are often used to makes things less definite. Most things aren’t black or white, they are something in between. Using double negatives people give the sense of things being less definite.
Some people even find double negatives graceful and elegant.
Read more: http://culture-2.webnode.nl/language-myths/
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