There is a subtle but important clue to the genesis
of such an unhelpful taxonomy in Grice's original
article [Grice 75]. Grice says that his categories
are echoing Kant" (p. 45). This clearly refers to
Kant's theory of categories, which classied declarative
statements along four dimensions: Quantity, Quality,
Relation, and Modality. This makes it clear why
Grice's taxonomy does not t the discourse phenomena
it was supposed to describe; it has been borrowed as a
whole from a pre-linguistic, philosophical classication
of statements! Taxonomies from one domain simply
cannot be transferred wholesale to another and retain
any usefulness. It is interesting that this attribution
has never been quoted, to my knowledge.