These [noun, verb, pronoun, adverb, preposition, conjunction] are the Indo-European “parts of speech:” that is to say, the main classes of words, having restricted application and definite connection, into which the holophrastic (‘equivalent to a whole phrase) utterances of a primitive time have by degrees become divided; the separated parts, members, of what was once an undistinguished whole.
- Whitney (1875), Pag. 209