Most significant policy issues facing humanity reach across national borders. Consequential political decisions with cross-national effects are frequently made by states, non-state organisations, and corporations. Under these circumstances, it is widely acknowledged that it is important to conduct deliberation at the global level. Below this shallow agreement, however, lies deep disagreement about a crucial question: how, if at all, is it morally permissible for deliberation to result in a set of international laws and rules that are imposed on a world (...) population which is deeply pluralistic in its moral and political attitudes? When the equivalent question is asked within the confines of a political community, one prominent answer is by reference to a standard of public reason. While there is a large literature about public reason at the domestic level, the literature on global public reason is comparatively underdeveloped. The paper addresses this lacuna in two ways. First, it motivates the global public reason project, and conceptualises the nature of the challenge that accounts of global public reason face. Second, it demonstrates that, by their own evaluative standards, existing accounts of global public reason are unable to satisfy both desiderata simultaneously, being either too ‘thick’ or too ‘thin’. (shrink)