For property rights to be upheld, people need to be able to judge how ownership is established. Previous research suggests that people may judge that the first person to possess an object establishes ownership over it. This article proposes and tests an alternative account, which claims that people decide who owns an object by judging who was probably necessary for the object to be possessed. Participants read stories in which one character pursues an object (e.g., an animal being hunted, a gem jutting out of a high cliff), which a second character then captures. Judgments about which character owns the object depended on which character was plausibly necessary for capturing the object. The findings support the "necessary for possession" account and suggest that people's judgments about ownership likely depend on counterfactual reasoning or on processes akin to those used to make judgments about causality.