1. The laws of nature are the same in all inertial frames of reference.
2. The speed of light is the same in all inertial frames of references.
Einsteins first postulate, the Principle of Relativity (that natures laws are the same in all inertial frames), may be regarded as the keystone of the idea of invariance, and indeed might better have been named the Principle of Invariance. The theory of relativity has brought to science both more relativity and more invariance. According to the old idea that the Galilean transformation represented the right way to relate the observations of two observers in relative motion, mechanics was invariant (its laws were the same in all inertial frames), but electromagnetism was not invariant. The Principle of Relativity asserts that all laws of nature, those known and those yet to be discovered, are the same in all inertial frames of reference. In this sense relativity is a theory of theories, imposing requirements even on theories not yet formulated. The straitjacket of the Principle of Relativity has been a valuable tool for more than a century and continues in that role as scientists search for new laws of the submicroscopic world. It has so far met only with success, and will, like the principles of energy conservation and momentum conservation, remain a pillar of physical science until experimental evidence forces its rejection or modification.
The postulate of the constancy of the speed of light is itself an invariance law. Indeed if one chose to call the statement “Light travels at the constant speed c” a law of nature, then Einsteins second postulate (that light travels at the same speed with respect to all observers) could be dispensed with and the entire theory of relativity would follow from the Principle of Relativity alone. This is entirely a matter of taste. It is probably better to let postulate 2—light’s constant speed—stand as a reminder of the particular significance of the speed of light in relativity. (There is one other important invariant quantity in special relativity, the four-dimensional spacetime interval, which replaces the old ideas of separately invariant distances and times.)