I thought it might be of some interest if I posted here a contribution I have made to a discussion elsewhere. It consists of the six following propositions, which I believe to represent, more or less, current linguistic orthodoxy.
1. A language is a set of dialects having certain common features.
2. Grammar describes the way in which a dialect allows units of meaning to be put together to make words (morphology), and the way it allows words to be put together to make sentences (syntax).
3. The only admissible evidence for determining a dialect’s morphology and syntax is that obtained from NANS (normal adult native speakers).
4. A construction that a NANS would never use is by that fact alone ungrammatical, but the opposite is not necessarily the case.
5. The standard variety of a language is one dialect among many. It may be of great political, economic and social importance, but it is not linguistically superior to any other. The grammar of nonstandard dialects, like the grammar of the standard, is internally consistent.
6. The grammatical features of a standard dialect, like the grammatical features of all dialects, are matters of objective fact. Personal opinions and tastes are irrelevant.