 I am very grateful to the many people who have provided feedback on this entry since its initial publication in 2009. I would like to particularly thank the editors of this entry for their work: Luca Ferrero (2020), and John Martin Fischer (2009, 2014). For additional feedback on the most recent revision (2020), I would like to thank: Vanessa Carr, Romy Jaster, Simon Kittle, and Barbara Vetter. I unfortunately do not have a comprehensive record of people who provided thoughts on earlier versions, but they include: John Bengson, Helen Steward, Kadri Vihvelin, and Ann Whittle. Thanks finally to the SEP Editors and Michael Nelson for their assistance.