Disimplicature

Speranza

Grice once developed an account of the meaning of `ought', and the distinction between weak necessity modals (`ought', `should') and strong necessity modals (`must', `have to'). 

He argues that there is nothing specially ``strong'' about strong necessity modals per se: uses of `Must p' predicate the (deontic/epistemic/etc.) necessity of the prejacent p of the actual world (evaluation world). 

The apparent ``weakness'' of weak necessity modals derives from their bracketing whether the necessity of the prejacent is verified in the actual world. 

`Ought p' can be accepted without needing to settle that the relevant considerations (norms, expectations, etc.) that actually apply verify the necessity of p. 

Grice calls the basic account a modal-past approach to the weak/strong necessity modal distinction (for reasons that become evident). 

Several ways of implementing the approach in the formal semantics/pragmatics are critically examined. 

The account systematizes a wide range of linguistic phenomena: it generalizes across flavours of modality; it elucidates a special role that weak necessity modals play in discourse and planning; it captures contrasting logical, expressive, and illocutionary properties of weak and strong necessity modals; and it sheds light on how a notion of `ought' is often expressed in other languages. 

These phenomena have resisted systematic explanation. Grice briefly considers how linguistic inquiry into differences among necessity modals may improve theorizing on broader philosophical issues.

REFERENCES

GRICE, Aspects of reason

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"Does 'ought' conversationally implicate 'can'?