Acknowledgments
We would like to thank Simon Barker, Sami Paavola, Nancy Cartwright, Julian Reiss and Sarah Wieten for their comments on earlier drafts, as well as the Library Staff at Maine Medical Center, Georgia Wark, Dina McKelvy and Heather Kemp, for help with the bibliography. The paper has been presented at the 2016 Meeting of the Nordic Network for Philosophy of Science and to the CHESS research group at Durham University. The discussion at both events was very helpful. The paper is fully joint work.