The Rationale for the Present Book Perhaps the most critical problem facing present-day particle physicistsis to delineate the relationship between classical and quantum systems. This relationship has many facets. Particle-waveduality is one. The concept of the point particle is another. And theconcept of particle mass is yet another. The electron, as the lightest of the charged particles, represents a fundamental "ground state",and many of the essential problems in the murky area between the domainsofclassical and quantum physics can be brought into focus by studyingjust this one particle. Thus the present book is centered on questions that arise in connection with the electron, and in particular with its mass, which has remained an unsolved, and indeed almost unexplored, mystery. Each student ofphysics, beginner and professional alike, has to fashion for himselfa way of thinking about the electron. If, after reading this book, the reader views this topic somewhat differently than before, the efforts of the author will have been amply rewarded. When physicists were confronted with the properties of the electron, they made a conceptualleap into the unknown: they concluded that the electron does not obey classical laws with respect to mechanics (as connected to the spin of the electron), and also with respect to electrodynamics (as connected to the magnetic moment of the electron).