Let us see where the line between "actual" and "possible" words might run. Obviously "arrival" is an actual word of English and "*derival" is not an actual word, though it is a possible word because it is formed by a morphological process which could apply to the verb "derive" but happens not to apply to it. [...] The set of "possible words" is obtained by dropping all idiosyncratic restrictions on the morphological and phonological rules of the lexicon.
- Kiparsky (2004), Pag. 119