What's the Deal with Double Negatives in English?

The double negative rule is often said to be grounded in maths. The logic goes that if you add a minus and a minus together, you get a plus. And, of course, in maths, if you subtract a negative, you get a positive. Somewhere down the line, someone decided this is a good way to also create grammar to guide the use of language. And so, doubling your negations is supposed to result in a positive statement. In the example given above, the “ain’t” and “nothing” are supposed to add up to the person, in reality, having something.