Originally published in 1965, the aim of this book was to challenge the dualistic view of physics, that is, the assumption that beams of electrons consist of discrete particles and of waves. Lande argues that this dualistic view is unnecessary, not only on methodological grounds but also from the standpoint of physics. Lande sets out to point out that there are faults in the purely physical arguments, which have led to the dualistic doctrine and shows that by making use of the quantum rule for the exchange of linear momentum, established by W. Duane in 1923, wave-like phenomena can be fully explained on a unitary particle theory of matter. Chapters cover a variety of subjects and range from 'Dualism versus quantum mechanics' to the 'Origin of the quantum rules'. Appendices are included for reference. This book will be of value to students and scholars of the history of physics.