Relativity and the nature 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. TOC:Introduction. Deriving All Mathematical Implications from the Conceptual Picture Obtained from the Analysis of Galileos Relativity Principle. Analysis of the Kinematic Relativistic Effects to Reveal What Their Physical Meaning Is. The Nature of Spacetime. Beyond the Physics of Spacetime