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 position that we shall adopt here is that the set of actual words of a language, in the second, linguistically significant sense where it is an unbounded set, is recursively defined by the lexicon.
- Kiparsky (2004), Pag. 119