H. points out that there is a contradiction between Whorf's hypotheses that (i) the structure of language determines the structure of thought, and that (ii) there are in all languages covert grammatical categories that are not less important than the overt categories. He observes that in English, where the notion of presupposition is a covert category, it plays a purely grammatical role whereas it does not in Hua, though that language marks presupposition morphologically. According to H., the data, which confirm Whorf's second hypothesis, undermine the first one.