Abstract
There is a difference between the conditions in which one can felicitously assert
a ‘must’-claim versus those in which one can use the corresponding non-modal claim. But it
is difficult to pin down just what this difference amounts to. And it is even harder to account
for this difference, since assertions of 'Must ϕ' and assertions of ϕ alone seem to have the
same basic goal: namely, coming to agreement that [[ϕ]] is true. In this paper I take on this
puzzle, known as Karttunen’s Problem. I begin by arguing that a ‘must’-claim is felicitous
only if there is a shared argument for its prejacent. I then argue that this generalization, which
I call Support, can explain the more familiar generalization that ‘must’-claims are felicitous
only if the speaker’s evidence for them is in some sense indirect. Finally, I sketch a pragmatic
derivation of Support.