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The contrast between imperatives and promissives brings out the issue well. These two types are functionally very similar: An imperative places a requirement on the addressee, while a promissive places a requirement on the speaker. Yet imperatives are apparently universal (and at least extremely common), while promissives are extremely rare. It does not seem easy to give a functional explanation for this contrast, and so in is reasonable to inquire into whether an explanation in terms of syntactic or semantic theory is possible. Clauses may of course be categorized in a wide variety of ways, and so it is a necessary precondition to the study of clause type systems to have a definition which makes clear which types of types we have in mind. Sadock the major types are universal , whereas the others have noteworthy typological properties. Saying that clause types form a closed system is to say that every sentence is a member of one and only one type. This implies , for example, that the indicative mood is not a clause type in the relevant sense, since root declaratives and interrogatives are typically both declarative/interrogative and indicative. The concept of force most frequently invoked, especially in the syntax literature, is that of illocutionary force, but this is not accurate. Illocutionary force is a communicative-intentional concept, so that (2), used in its stereotypical way, has the force of requesting, not asking.
Paul Pörtner (Thu,) studied this question.
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