IT GIVES ME pleasure to acknowledge my indebtedness to Prof. Clifford M. Will, the leading specialist on experimental gravitation, and to Prof. Jacob Bekenstein, the well-known expert on the theory of relativity, for reading my entire manuscript and for their invaluable critical remarks. I am also grateful to the two anonymous referees of the draft for their constructive critical comments. I thank my friends and colleagues Profs. Abner Shimony, Yuval Ne’eman, Lawrence Horwitz, Nissan Zeldes, and Jacob Levitan for enlightening discussions. Finally, I express my gratitude to Dr. Trevor Lipscombe, the physics editor of Princeton University Press, and to Ms. Evelyn Grossberg, the copyeditor for Princeton Editorial Associates, for their fruitful cooperation.