', which moves uniformly and rectilinearly with respect to
, is also an inertial system. Hence, since the laws of Newtonian mechanics are based on inertial systems, it follows that all such systems are equivalent for the description of the laws of mechanics: no one system amongst them is unique, and we cannot define absolute motion or rest; any systems moving with mutual rectilinear uniform motion may be regarded as being at rest. Mathematically, this means that the laws of mechanics remain unchanged in form for any transformation from one set of inertial axes to another.
The development of electrodynamics and the phenomena of radiation generally showed, however, that the laws of radiation in one inertial system did not preserve their form when referred to another inertial system:
and
' were no longer equivalent for the description of phenomena such as that of light passing through a moving medium. This meant that either there was a unique inertial system enabling us to define absolute motion and rest in nature, or that we would have to build up a theory of relativity, not on the inertial law and inertial systems, but on some new foundation which would definitely ensure that the form of all physical laws would be preserved in passing from one system of reference to another.
This dilemma arose out of the conflicting results of two experiments, viz. Fizeau's and Michelson and Morley's.