Whorf (1956: 88ff) apud Halliday & Matthiessen (1999: 27):
A covert category is marked, whether morphemically or by sentence pattern, only in certain types of sentence and not in every sentence in which a word or element belonging to the category occurs. The class membership of the word is not apparent until there is a question of using it or referring to it in one of these special types of sentence, and then we find that this word belongs to a class requiring some sort of distinctive treatment, which may even be the negative treatment of excluding that type of sentence. This distinctive treatment we may call the reactance of the category.