When judgement was called for in Hamlet’s duel with Laertes, the official result was Nothing neither way. This must have given a headache to generations of teachers trying to impress on their students that they shouldn’t protest I never done nothing. Why did the greatest writer in the language transgress so?
The answer is that it was perfectly correct to say that in Shakespeare’s day, just as it’s perfectly correct to say in French ce n’est pas faux, even though it contains two negative elements.
The No Double Negative rule was the result of grammarians trying to apply mathematical logic to language. This would be a dubious activity anyway, but in doing so they committed the elementary logical error of assuming that, just because two things are called the same, they must be the same.
The argument’s simple: if you multiply -2 by -2, you get +4; therefore, if you apply one grammatical negatives to another, the result is a positive statement. This doesn’t hold water, though. A negative number isn’t just the absence of the positive, it’s a mirror image, and a mirror image reflected in another mirror ends up the right way round.
This isn’t true of a grammatical negative. If it were, the statement I didn’t take a step forward would actually mean I took a step backward. In fact, a negative here is simply a statement of nothing – though in relation to a specific nothing. The mathematical equivalent would be multiplying by zero, since multiplying any number by zero simply negates it, producing zero.
So what do we get from 2x0x0? Two? Four? Of course not – it’s exactly the same as 2x0. (Now I’m going to get a mathematician protesting that there’s some theoretical difference between the two sums, but that’s just splitting hairs. The result is the same.) In the same way, a negative negativing another negative should still give us a resounding negative.
So what does this prove? Well, nothing really, apart from as a cautionary tale against trying to apply logic to anything as illogical as language. The English language is what it is – chaotic and inconsistent – and the rule against a double negative is part of that now. Perhaps, though, we could feel less superior next time we hear someone breaking that rule. It was good enough for Shakespeare.