 We would like to thank Joe Dewhurst, James Nguyen, Alexander Reutlinger, Collin Rice, Dunja Šešelja, and Paul Teller for helpful comments on the drafts of the revised version in 2019. When writing the original version back in 2006 we benefitted from comments and suggestions by Nancy Cartwright, Paul Humphreys, Julian Reiss, Elliott Sober, Chris Swoyer, and Paul Teller.