© 2015, © The Author(s) 2015. Carens’ book covers a wide range of issues concerning the ethics of immigration, and although he is best known as a theorist of open borders that argument takes up a relatively small part of the book, and is indeed a small part of his writing on immigration. In this essay, I examine the relationship the radical arguments for open borders, and the more contextual arguments about specific issues such as birthright citizenship, naturalisation and temporary workers which fall short of that radical position. This relationship is complex, and reveals that, although this is a highly readable and accessible book, it rests upon a highly complex method of doing political theory. I critically examine that method and look at the problems it raises for the relationship with political theory and activism. I ask what role the radical arguments for open borders are supposed to play in the public sphere, and, in the end, what role political theorists are supposed to play in that sphere.