On the Impossibility of Detecting Uniform Motion -- Exploring the Internal Logic of Galileo’s Principle of Relativity -- Relativity in Euclidean Space and in Spacetime -- Relativity and the Dimensionality of the World: Spacetime Is Real -- Quantum Mechanics and the Nature of Spacetime -- The Nature of Spacetime and Validity of Scientific Theories -- Propagation of Light in Non-Inertial Reference Frames -- Calculating the Electric Field of a Charge in a Non-Inertial Reference Frame -- Inertia as a Manifestation of the Reality of Spacetime.The most important question that is addressed in this book is "what is the nature (the ontological status) of spacetime?" or, equivalently, "what is the dimensionality of the world at the macroscopic level?" The answer to this question is developed via a thorough analysis of relativistic effects and explicitly asking whether the objects involved in those effects are three-dimensional or four-dimensional. This analysis clearly shows that if the world and the physical objects were three-dimensional, none of the kinematic relativistic effects and the experimental evidence supporting them would be possible. The implications of this result for physics, philosophy, and our entire world view are discussed.