This book revisits Einstein's postulate of the invariance of the speed of light, focusing on the corresponding train thought experiment. It shows how a close analysis of the supposed relativity of the simultaneity of light beam emissions brings us to the principle of the relativity of simultaneity at the physical level. However, in the objection about a spacecraft and a missile presented here, this principle turns out to be self-contradictory. The present logical and mathematical considerations are liable to challenge the validity of the second postulate of Special Relativity. Recognition of the results of this analysis could revolutionize our conception of spacetime.