 The authors owe thanks to a group that met in Berlin in July, 2014, to discuss a draft of the original version of this entry, including Dina Emundts, Eckart Förster, Gunnar Hindrichs, Charles Larmore, Paul Redding, Robert Stern, and Tobias Rosefeldt; we owe special thanks to Larmore for his numerous and detailed comments on that draft and to Stern for his generous assistance with the bibliography. We also owe thanks to Justin Broackes for his participation in the seminar we gave at Brown University in Spring and Fall 2013 where we also discussed much of this material. This revised version owes thanks to the participants of another seminar we gave at Brown in Spring 2020 and to helpful comments by Allen Wood.