[...] the phonetic changes seem to be the most important and the most removed from direct observation. The factors which lead to these phonetic changes are probably exceedingly complex and no doubt include the operation of obscure symbolism which define the relation of various age groups to each other.
- Sapir (1933a), a pag.23

Not all phonetic changes [...] can be explained in terms of social symbolism. It seems that many of them are due to the operation of unconscious economies in actualizing sounds or combinations of sounds.
- Sapir (1933a), a pag.23

The most impressive thing about internal phonetic change is its high degree of regularity. It is this regularity [...] that is more responsible than any other single factor for the nviable degree of exactness which linguistics has attained as a historical discipline.
- Sapir (1933a), a pag.23