Language, as defined by the Oxford English Dictionary, is difficult to pin down. Even this concept has multiple meanings; “The system of spoken or written communication used by a particular country, people, community, etc., typically consisting of words used within a regular grammatical and syntactic structure,” “the vocal sounds by which mammals and birds communicate,” “A means of communicating other than by the use of words,” (emphasis mine) and even “The style of a literary composition; (also) the wording of a document, statute, etc.” There are more meanings to the word, which is perhaps the problem in a nutshell: Language cannot be described without the use of itself. That is to say, one must use language in order to describe language. Any discussion of the use language must, therefore, begin with the acknowledgement that language is itself as nebulous as any other term.
It is not the intention of this essay to gaze too deeply into the abyss that lurks behind the assigned meaning(s) of a word, but it illustrates an inherent instability in the practice of language that has been noted by other writers and will continue to be noticed and commented upon; it will be the case that writers and indeed anyone who speaks (any) language(s) will forever have to wrestle with language in order to convey the meaning they want to convey, or at least a meaning that can be called close enough for comfort (or government work, or your own phrase of choice). This still does not come any closer to saying what language is, though it does come closer to saying why it's important; language is perhaps the best possible (though imperfect) method for transmission of thoughts short of telepathy.
Language allows the world to if nothing else make slightly more sense than it would otherwise, and allows the people of the world to make a kind of sense of one another. Language makes sense of things, categorizes things, expresses what could not be otherwise expressed. This is the importance of language, this is why language matters. Language's rather nebulous nature, however, means that using language to communicate complex ideas takes a special amount of care and precision. True mastery of language comes from being able to communicate one's ideas clearly and effectively. To say what needs to be said in a way that the meaning is left to little doubt, to utilize all the tools which have been created for the management of language over centuries of practice (metaphor, allegory, etc.), to use the rules of style and form that had been agreed upon for forms of fiction and essay and poetry—that is what separates a writing novice from a true craftsman, so to speak. In the beginning, anyway...
