 The author would like to thank Stephan Hartmann, Wendy Parker, Paul Teller, Alessandra Basso, Sally Riordan, Jo Wolff, Conrad Heilmann and participants of the History and Philosophy of Physics reading group at the Department of History and Philosophy of Science at the University of Cambridge for helpful feedback on drafts of this entry. The author is also indebted to Joel Michell and Oliver Schliemann for useful bibliographical advice, and to John Wiley and Sons Publishers for permission to reproduce excerpt from Tal (2013). Work on this entry was supported by an Alexander von Humboldt Postdoctoral Research Fellowship and a Marie Curie Intra-European Fellowship within the 7th European Community Framework Programme. Work on the 2020 revision of this entry was supported by an FRQSC New Academic grant, a Healthy Brains for Healthy Lives Knowledge Mobilization grant, and funding from the Canada Research Chairs program.