Exasperating is the oftness and drama with which narrow-minded notions about language are declared and repeated, even honored. Through misinformed propagation, often at the hands of educators, they even become commonplaces, 'truisms', which are too often accompanied by smug self-satisfaction and eager denunciation. They become value judgments, and for all that, they are frequently mistaken. Underneath the surface, sometimes deep down, and sometimes more shallowly, bides a social, class or racial judgment. The existence of different social dialects is perfectly natural, and the derision of someone's speech is an easy way to deride the person, and an easy way to disguise (whether knowingly or not), a derision for their class or social station, or ethnicity.
Probably the chief causes of the problem lie in ignorance of: 1. Other languages; 2. The history of our own language, 3. Natural variability across dialects, and the nature of a Standard Dialect; The processes and effects of language contact.
Knowledge of, and reflection on, another language will quickly dispell many ideas about how language itself must work. It is far easier to assume that the rules of the standard dialect of one's own mother tongue are immutable and perfectly logical - inevitable, even - if one is sheltered from the structure and rules of another language. A knowledge of French, for instance, would call into question the assumption that the double negative is inherently wrong, and that its use must signify either bad English and poor education, or irony. In French, the double-negative reinforces or strengthens the negation, rather than negates it (creates a positive), as proscriptive grammarians of English insist it must needs do. Middle English, and to a lesser extent, Old English, used it as well (even sometimes triple-, or quadruple-negatives).
In the prologue to his Canterbury Tales, Chaucer says of the Knight:
"He nevere yet no vileynye ne sayde/In al his lyf unto no maner wight,"
whereas modern proscriptive grammar would demand we amend to:
"He never yet any vilainy has said/In all his life, unto any manner [of] wight (creature/man)."
Yet, the meanings of the original and the "corrected" version are identical. What's more, non-Standard modern English often uses the double-negative, with little confusion as to whether a strengthening or a 'positive' is intended. Clearly, the statement, "That is not untrue," does produce, in its round-about way, a positive ("That is true"), though the rhetorical effect is not identical with simply converting to a direct and positive formulation. They are not exactly the same. However, statements such as, "I didn't do nothing!", or, "I don't know nobody here," are fully functional, and leave no doubt as to whether the speaker is claiming to have done something and to know someone, or not. Even in our Modern English, two negatives make a positive only sometimes, and depending on context; even when they do make a positive, they rarely equate exactly with a simple positive formulation. It can be accepted that the double-negative is not included in (the) Standard (Dialect of) English, but in no wise can we assent to the mis-notion that it is inherently wrong, bad, or illogical. What more could one want by way of a blessing than the antiquity of, and honor accorded to, the 'Father of English poetry', Chaucer himself?
Notwithstanding any of the above, the double-negative is now widely scorned, and taken for a marker of low intelligence and personal worth, not unlike some other notable features. First among these, is probably the metathesized form 'aks' vs. 'ask' (also blessed by Chaucer; in fact, both forms existed simulatneously several hundred years earlier yet in the Old English period, as 'ascian' and 'acsian'), and other forms affected by the same process.
Close behind would likely be the inflection of verbs as either strong or weak, contrary to their Standard inflections, the which constructions are widely held to be abominations. So, where as Old English, Middle English, and all phases of Scots have forms along the lines of 'eode', 'yead', and 'gaed' (all equivalent to 'goed'), Modern English prohibits 'goed', and uses in its place, 'went'. So, in English, I go, but - I went, whereas in Scots, I gae, and I gaed; or, I gang, and I ganged.
Similarly, whereas Modern English 'teach' is inflected strongly in the past tense to 'taught', it could have been inflected as a weak verb, to 'teached' (or the Modern Scots form, 'teachit').
Just like the double negative, these kind of constructions are scorned and mocked widely, and they lend themselves easily to other, sleekit purposes, like social and ethnic prejudice. What's more, the judgments based on them are illusory, being based on a very small and circumscribed view and knowledge of the thing which is ultimately being judged: (a particular) language. As soon as one begins to grasp the nature of the way language evolves and develops over centuries, one begins to lose the self-righteous bearing of the proscriptive grammarian.
