  Thanks to Philip Ebert, Leon Horsten, James Ladyman, Hannes Leitgeb, David Liggins, Alexander Paseau, and Philip Welch for comments and discussion. Thanks also to an audience at the ECAP6 in Krakow, where parts of this material were presented. This article was written during a period of leave funded by an AHRC Research Leave Grant (number AH/E003753/1). I gratefully acknowledge their support.