Зачем существует система фонем?

System is one of the most important concepts of modern linguistics, including phonology, but it remains unclear in what sense the phonemes of a language form a system. It is apparent that the reference is not due to the fact that vowels and consonants function in series nor to the interplay between the prosodic and the segmental levels. Nowadays, phonemes are usually presented in the form of generative models, and the possibility to do so is understood as system's justification. Yet the results of such modeling testify to the researcher's ingenuity rather than to the organization of language. The role of the actual system, rather than such as is imposed on it by a linguist, comes to the foreground in diachrony. This role does not consist in producing symmetry (for instance, filling a case vide) or facilitating push- and drag-chains. System exists to prevent the changes of sounds and prosodemes from disrupting the process of communication; it even makes them unnoticed by the speakers. Only diachronic phonology, though it borrows its framework from general phonology, gives life to two basic concepts of linguistics: system and the distinctive feature. Change reveals their uses, which otherwise remain hidden. The conclusions in the present article are drawn from the material of Germanic languages, mainly English: The Great Vowel Shift and some compensatory processes, including such consequences of apocope as the circumflex and vowel lengthening. Throughout the article, special emphasis is put on the protective role of the system.