There are two types of variation that tend to obscure the distinctiveness of the different points in the phonetic pattern of a language. One of this is individual variation. [...] no two individuals have precisely the same pronunciation of a language, but [...] they aim to make the same sound discriminations [...] if the qualitative differences of the sounds that make up A's pattern from those that make up B's are perceptible to a minute analysis, the relations that obtain between the elements in the two patterns are the same.
- Sapir (1925), a pag.36