This chapter discusses the implications of a famous natural law category for questions of border-crossing migration, namely, the category of original common ownership of the earth, as used by Hugo Grotius and Immanuel Kant. Kant derives a cosmopolitan right of hospitality from it. Yet the debate on cosmopolitan right is still to address the ‘unbridgable gap’ (Benhabib) between its claim to temporary sojourn and the further claim, not covered by cosmopolitan right, to take up permanent residency. Some explanation is needed for how this gap came about and how to deal with it. The chapter argues that the bifurcation of cosmopolitan right results from the fact that Kant is drawing on two different sources from the Grotian tradition of original common ownership, two arguments which yield widely diverging entitlements. The first is the idea of an original possession in common, which is divisible. The second is the idea of a common heritage of mankind, which is indivisible. Both Grotian sources can be systematically developed and transformed from natural law into public cosmopolitan law. Although they cannot be merged in a fully satisfactory way, the chapter concludes by discussing different options for policies combining the two theoretical traditions in new ways.