 Many thanks to Lieven Decock, Wim de Jong, Iris Loeb, Venanzio Raspa, Stefan Roski, Maria van der Schaar, two anonymous reviewers, the SEP editor Ed Zalta, and to my students Koen Kramer, Remco Reisig and Jeroen Smid for helpful comments on an earlier version. Work on this entry has been supported by ERC Starting Grant TRANH Project No 203194.