Reduced interrogative clauses with a causal wh-word and a negative marker (like English ‘Why not’) can have (at least) two kinds of readings:

1. Factive reading:
   A: Mary didn’t sleep.
   B: (Really?) Why not?

2. Modal reading:
   A: Let’s sleep.
   B: (Sure,) Why not?

The paper outlines different properties of the two readings in terms of their interpretation and anaphoric behavior. The factive reading receives its interpretation in combination with a salient negative proposition in the context. (1) can be paraphrased as Why didn’t Mary sleep?. The modal reading receives a modal interpretation in relation to a salient antecedent in the context. The literal content of (2) can be paraphrased as Why shouldn’t we sleep? The modal reading is generally accompanied by a rhetorical question inference, like There is no reason we shouldn’t sleep in (2). The paper outlines properties of the two readings, which differ in their licensing conditions, anaphoric behavior, and cross-linguistic realization. It also provides an analysis of the interaction of the factive reading and negation based on a syntactic dependency between the negative remnant (why not) and negation in the ellipsis site.