Einstein's luminous scientific essay begins with a question that once haunted physics: if forces act across empty space, what fills the gap? From Newton's gravity to the wave nature of light, polarization, Maxwell's fields, Hertz's experiments, and Lorentz's refinements, he traces the rise and collapse of the luminiferous ether, showing how each theory tried to reconcile matter, electricity, and the electromagnetic field. Special relativity seems to banish ether altogether, yet Einstein argues instead for a deeper redefinition: space is not merely empty extension, but a physical structure with real properties, especially in the framework of general relativity. From there he turns to geometry, measurement, and the cosmos. Pure mathematics gains certainty only as formal axiomatics, while practical geometry must answer to rods, clocks, light, and experience. Euclidean assumptions give way to Riemannian space-time, and the universe itself may be finite yet unbounded, like a spherical surface with no edge. Along the way, Einstein asks whether gravity and electromagnetism can be unified, and whether quantum theory sets limits on that dream. This edition has been fully restored with modern typesetting, custom cover design.