 The author would like to thank a grant from the John Templeton Foundation for support in the most recent revision to this entry, as well as Beth Hannon for help in preparing revisions to an earlier version, and the European Research Council for financial support under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC Grant Agreement 284123. The editors would like to thank both Christopher von Bülow and Sally Ferguson for carefully reading the entry and identifying a significant number of typographical errors for correction. 