[…] there is another class, the interjections, which are not in […] the proper sense a "‘part’ of speech:” which are, rather, analogous with those all-comprehending signs out of which the rest have come by evolution. A typical interjection is the mere spontaneous utterance of a feeling, capable of being paraphrased into a good set expression for what it intimates: thus an ‘ah!’ or an ‘oh!’ may mean, according to its tone, ‘I’m hurt,’ or ‘am surprised,’ or ‘am pleased,’ and so on; only there is no part of it which means one of the elements of the statement while another part means another. Yet, such creatures of conventional habit in regard to expression have we become by our long use of the wholly conventional apparatus of language, that even our exclamations have generally a conventional character, and shade off into exclamatory utterance of ordinary terms.
- Whitney (1875), Pag. 209-210