 We want to thank Manula Adhihetty for his valuable work helping us edit the entry and completing the bibliography. We received excellent feedback on a previous draft of this entry from a number of colleagues and friends, including Giovanna Colombetti, Phoebe Ellsworth, Rebekka Hufendiek, Agnes Moors, Jesse Prinz, Jim Russell, Disa Sauter, Dan Shargel, Achim Stephan, Christine Tappolet, Fabrice Teroni and an anonymous referee. Fabrice also gave us very valuable comments on the previous version of this entry, for which we are very grateful.