Speech versus writing

Linguistics examines all forms of language, but the written word is considered at best an incomplete representation of a linguistic system. Linguists generally consider that more fundamental insights can be gleaned into the nature of language by analysing natural, spontaneous speech, rather than assuming the primacy of writing.
Languages have only been written for a few thousand years, but have been spoken (or signed) for much longer. The written word may therefore provide less of a window onto how language works than the study of speech, even assuming that the culture which the language forms part of has a writing system - the majority of the world's languages remain unwritten. Furthermore, the study of written language can play no part in investigating first language acquisition, since infants are obviously yet to become literate. Overall, language is held to be an evolutionary adaptation, whereas writing is a comparatively recent invention. Spoken and signed language, then, may tell us much about human evolution and the structure of the mind.
Of course, linguists also agree that the study of written language can be worthwhile and valuable. For linguistic research that uses the methods of corpus linguistics and computational linguistics, written language is often much more convenient for processing large amounts of linguistic data. Large corpora of spoken language are difficult to create and hard to find, and are typically transcribed and written. Additionally, linguists have turned to text-based discourse occurring in various formats of computer-mediated communication as a viable site for linguistic inquiry. Writing, however, brings with it a number of problems; for example, it often acts as a historical record, preserving words, phrases, styles and spellings of a previous era. This may be of limited use for studying how language is used at the present time.