A fundamental requirement for a comprehensive description of a language is the

Categorical imperative of language description

Describe your language in such a way that the maxim of your description could serve, at the same time, as the maxim of the description of any other language. (Lehmann 1989, § 3)

Lehmann, Christian 1989, “Language description and general comparative grammar”. Graustein, Gottfried & Leitner, Gerhard (eds.), Reference grammars and modern linguistic theory. Tübingen: M. Niemeyer (Linguistische Arbeiten, 226); 133-162. [download ]

Since languages are both alike and diverse, this condition is janus-headed:

1. On the one hand, the description must bring out the specific characteristics of this language.
2. On the other, where all languages are alike, the description must follow a universal model.

If the two requirements are not met, the description fails both on theoretical and on practical accounts:

A description that is structured by concepts which are alien to the language does not meet requirement #1. It fails theoretically and is scientifically uninteresting. It fails practically because one cannot learn or use a language by applying a description that does not fit its structure.

A description that does not meet requirement #2 is obscure, making the language appear as something strange and incomparable to other languages. It is not usable for general-comparative linguistics and consequently does not feed into general-comparative grammar and into the construction of a theory of language.

Consequently, the descriptive linguist needs a general framework which incorporates what is universal about languages and is adaptable to the specific traits of the given language.