Must φ is felicitous only if φ is not known

Epistemic modals, deduction, and factivity: new insights from the epistemic future

“…We argue that epistemic must is in fact underspecified with respect to strength, as by default allowed by its semantics. This connects to, and improves on, recent work by Goodhue (2017).…”

“…See alsoGoodhue (2017) for a recent discussion of weakness (cf Karttunen 1972;Kratzer 1981Kratzer , 1991Kratzer , 2012. Giannakidou & Mari 2018, a.o.)…”

What ‘must’ adds

“…See von Fintel andGillies (2010), alsoVeltman (1985),Kratzer (2012a),Matthewson (2015),Ozturk and Papafragou (2015),Lassiter (2016),Gunlogson and Gregory (2016) andSherman (2018) Giannakidou and Mari (2016). andGoodhue (2017) both argue that this empirical generalization is better explained in terms of partial knowledge, rather than indirectness; but they both still, as far as I can tell, endorse the generalization when understood in a sufficiently broad way, as intended here. As I emphasize in a moment, I am leaving the notion of indirectness intentionally broad and vague here; in what follows, I will propose one particular way of cashing it out, a way which, of course, not all these writers would necessarily endorse.…”

“…These cases show a kind of context-sensitivity in which something like a skeptical scenario has been raised, making the inference from a given set of premises to a conclusion less secure than it normally is. This may make it look like the sense of indirectness relevant for 'must' patterns with knowledge ascriptions, something that Giannakidou and Mari (2016) and Goodhue (2017) have indeed argued for. But as Goodhue points out, there are other forms of context-sensitivity relevant to 'must' that do not seem to have anything to do with knowledge.…”

“…22 Giannakidou and Mari (2016) and Goodhue (2017) have argued that this 'must' is not genuinely epistemic, but instead is a "logical" or "deductive" 'must'. It is hard to rule out an ambiguity thesis, but it is obviously inelegant to multiply modal flavors further than we need to.…”