“Everyday language” is not innocent or neutral. It is the language of Western metaphysics, and it carries with it […] a considerable number of presuppositions […] which are knotted into a system.
Jaques Derrida, On Grammatology.
For most of people born and raised in a literate environment, thinking of a language becomes an equivalent of thinking of its writing: despite the fact that writing, as we know it now, is historically much more recent than orality; despite we don´t learn our mother tongue by means of writing; despite the predominance of oral language in our lives, writing is often the basis of any reasoning when it comes to reflect on language.
This “writing-centrism” has made us believe that language has an immutable presence, that it exists regardlessly people who speak it, just like writing. However, it doesn´t. Homogeneous societies are ideal abstractions: variation is a constitutive part of every language.
Unfortunately, the only reference most people often have about language is the one they have studied at school or while learning a second language: in both cases, language is taught as a prescriptive grammar concerned with rules for correct usage.
However, there is no such thing as correctness neither is there and absolute truth in what grammar explains: prescriptive grammar is merely one among hundreds of theories about language (not precisely the most popular between linguists) and it´s been developed for didacticism´s sake, after making a decision on which one, out of millions of slangs, is going to become the standard language.
Why is that, if no society has ever been homogeneous in terms of slangs, there is such a great effort made to keep this idea?
Because language is, among other things, a tool of power. (Click here for another post on the subject).
What is claimed to be the “correct language” is nothing more than just another slang (not that surprisingly it´s often the most powerful people’s slang), chosen to be the archetype of the language and imposted via educational system onto the rest of people. Behind every grammar there is a linguistic policy. It is normally taught as neutral to evade controversies on why one specific slang should be more adequate than others to represent the whole community.
To criticize one specific slang in terms of incorrectness is simply to claim that your own slang (and this always means your own belonging group) is better than others to represent the society as a whole, but there is no objective validation for that since there is no objectivity in grammar.