Fact-dependent policy disagreements and political legitimacy # 1. Introduction We sometimes face political disagreements that arise from divergences about the non-normative factual assumptions that underlie the justification of out policy choices. The main question in this paper is what political legitimacy requires in such cases, or indeed whether there are defensible answers to that question. A prominent case is the controversy over climate policy, insofar as this dispute can be traced back to disagreements about whether climate is changing, whether there is a significant anthropogenic cause to climate change, and # Earlier versions of this material were presented at Copenhagen University, Tilburg University, Oslo University and Aarhus University. I would like to thank all audiences for comments and discussion. Special thanks to Amelia Godber, Christian Rostbøll, David Estlund, Fabienne Peter, Jacob Elster, Kai Spiekermann, Karin Joench-Clausen, Martin Marchmann, Philip Pettit, and Wlodek Rabinowicz for suggestions. Support for this work from the Velux Foundation and the Carlsberg Foundation is gratefully acknowledged. A precursor of the paper is published in the ARENA conference proceedings, see (Kappel 2014).