 Bradley Armour-Garb and James Woodbridge joined Daniel Stoljar as co-authors of this (2021) version of this entry. Nicolas Damnjanovic was a co-author with Daniel Stoljar on the previous (2010) version. Daniel Stoljar would like to thank Damnjanovic for his contributions to the 2010 version, as well as Stewart Candlish, James Chase, Jakob Hohwy and Huw Price for help with the original (1997) version of the entry.