The fundamental physical and mathematical concepts which underlie the path integral approach to quantum mechanics were first developed by R. P. Feynman in the course of his graduate studies at Princeton, although more fully developed ideas, such as those described in this volume, were not worked out until a few years later. These early inquiries were involved with the problem of the infinite self-energy оf the electron. In working on that problem, a “east-action” principle using half advanced and half retarded potentials was discovered. The principle could deal successfully with the infinity arising in the application of classical electrodynamics.