This essay argues that a restatement of Thomistic natural law reasoning is increasingly necessary in jurisprudential debate about international law. Mindful of Pope John Paul II's call for a renewal of international law, the essay engages with the present-day tension between Morgenthau-type realism and neo-Kantian discourse-oriented cosmopolitanism. The essay addresses whether the former is sufficiently realistic in our global 21st century context, and whether the latter is adequately cosmopolitan. Attention is drawn to Aquinas's understanding of the relation between custom, consent (...) and political authority in order to expose some of the limits of present-day statism, and to suggest that Thomistic natural law reasoning is, potentially at least, better able to cope with the intractable disagreement that characterises 21st century global relations than some forms of neo-Kantian jurisprudence. (shrink)