“Must” implies “can”

We consider the thorny issue of whether ascribing to an agent the obligation to φ implies that it is possible for the agent to φ. Traditionally, this issue has been interpreted as whether “ought” implies “can”. But another linguistic interpretation is available, namely, whether “must” implies “can” (MIC). We show that “must” does imply “can” via a convergent argument. First, we prove MIC from a well-established theory of modality in natural language, namely, that proposed by Kratzer. Second, we present novel acceptability judgement studies showing that MIC predicts and explains the linguistic behaviour of native English speakers.