 When a person, S, does something, what is it for her to have the ability to do otherwise? For help we might look to alternative locutions: S can do otherwise; S has the power to do otherwise; this is possible for S; the world in which S does otherwise is accessible to S; doing so is up to S; this is open to S. But without further elaboration, these aren’t much more than synonyms for what’s to be explained. No theory of ability is adopted here, and for the most part we will rely on judgments about ability in particular cases. But a few general issues are worth flagging. (See also  §4.2.1  and the entry on  abilities.)