Ability and possibility

According to the classical quantificational analysis of modals, an agent has the ability to perform an act iff (roughly) relevant facts about the agent and her environment are compatible with her performing the act. The analysis faces a number of problems, many of which can be traced to the fact that it takes even accidental performance of an act as proof of the relevant ability. I argue that ability statements are systematically ambiguous: on one reading, accidental performance really is enough; on another, more is required. The stronger notion of ability plays a central role in normative contexts. Both readings, I argue, can be captured within the classical quantificational framework, provided we allow conversational context to impose restrictions not just on the “accessible worlds” (the facts that are held fixed), but also on what counts as a performance of the relevant act among these worlds.