Legal systems divide the world into persons and property, treating animals as property. Some animal rights advocates have proposed treating animals as persons. Another option is to introduce a third normative category. This raises questions about how normative categories are established. In this article I argue that Kant established normative categories by determining what the presuppositions of rational practice are. According to Kant, rational choice presupposes that rational beings are ends in themselves and the rational use of the earth's resources presupposes that human beings have rights. I argue that rational choice also presupposes that any being for whom things can be good or bad must be regarded as an end in itself, and that the use of the world's resources presupposes that any being who depends on those resources has rights. Although the other animals do not engage in rational practice, our own rational practice requires us to give them standing.