This book arose from the proceedings of two small conferences: one held in Minneapolis under the auspices of the Minnesota Center for the
Philosophy of Science, the other held in Andover under the auspices of the Boston University Institute of Relativity Studies. The similarity of
topics and the overlap of participants decided the editors to join forces and include the material from both conferences in a single volume. Indeed, one paper in this volume, the first by Howard Stein, was presented and discussed at both conferences. Further details about the conferences and a list of the participants will be found in the Appendix to this Preface. We would like to thank all the participants for their presence and their contributions. Special thanks are due the speakers for their patience in awaiting the printed version of the proceedings.