 We would like to thank Bruno Verbeek and Christopher Morris whose earlier version of this entry helped shape some of our thinking on the issues discussed here. We would also like to thank the many colleagues with whom we’ve discussed our presentation of the material discussed here, including especially colleagues at Chapman University and the University of Arizona. Finally, we would like to thank the editors for their patience with us during the writing process, and, especially, the anonymous referee whose substantial feedback on earlier drafts improved the final product.