Our conventional classification of words into parts of speech is only a vague, wavering approximation to a consistently worked out inventory of experience.
- Sapir (1921), a pag.117

[...] we might go on examining the various parts of speech and showing how they not merely grade into each other but are to an astonishing degree actually convertible into each other.
- Sapir (1921), a pag.118

[...] the ' part of speech' reflects not so much our intuitive analysis of reality as our ability to compose that reality into a variety of formal patterns. A part of speech outside of the limitations of syntactic form is but a will o' wisp.
- Sapir (1921), a pag.118

[...] no logical scheme of the parts of speech - their number, nature, and necessary confines - is of the slightest interest to the linguist. Every language has its own scheme. Everything depends on the formal demarcations which it recognizes.
- Sapir (1921), a pag.119

No language wholly fails to distinguish noun and verb, though in particular cases the nature of the distinction may be an elusive one. It is different with the other parts of speech. Not one of them is imperatively required for the life of language.
- Sapir (1921), a pag.119