“Who gets what?” is one of the central questions of political philosophy; “why?” is the other one. These questions about the ownership of property have been debated intensively within political and moral philosophy. One of the fundamental philosophical divides regarding property concerns whether property rights are the products of human convention or are in some sense pre-conventional or “natural.” One prominent view holds that people have property rights even in a state of nature before the development of social agreements or conventions. Such natural property rights are independent of any convention and are to be contrasted with “artificial” rights that only exist because of some convention. To make progress on this question, our research turns to the methods of cognitive science to examine the psychological and cultural bases for ownership. Previous work on the moral/conventional distinction indicates that people do not treat stealing property as conventional (e.g., Nucci and Turiel 1993; Tisak and Turiel 1984; Dahl and Waltzer 2020). However, these studies explicitly assume that one person owns the property that another steals. This doesn’t help us understand if property rules are generally considered to be conventional. Across several experimental studies, we find good evidence for thinking that property norms are conventional rather than "natural." Even so, there also seem to be important restrictions on what kinds of conventions can be genuine property norms and rights. This, we argue, is good evidence in favor of a generally Humean theory of property.