Einstein"s Relativity Principle applies to both mechanical and electromagnetic phenomena.
• Understanding Einstein's Relativity Principle: "Given any two inertial observers in uniform relative motion, both are equally entitled to consider themselves to be 'at rest'."
• Einstein based special relativity on Principles 1 and 2, which are both natural and easy to believe.
• Logically, Principles 1 and 2 imply Principle 2*. It is the role of special relativity to show how Principle 2* makes sense, but it requires a complete revision of our concepts of space and time.