In 1905 Einstein generalized the Galelian relativity principle [ applicable only to Newtons laws ] to the whole of Physics by postulating that :
Einstein's motivation was to avoid inconsistencies between Maxwell's equations and Galelian relativity. The Lorentz transformations must relate to actual space and time measurements.
Special Relativity abolishes the idea of absolute space [ for
example the ether ] and absolute time, but it leaves
unexplained the origin of inertial frames. Mach's principle
says that the inertial frames are determined be the rest of
the matter in the universe [ i.e. those that are non-
accelerating with respect to the rest of the universe ].
Einstein tried later to incorporate this idea into General
Relativity.n