This
principle (accepted by Galileo and Newton) was
extended by Einstein to include the laws concerning
the propagation of electro-magnetic radiations (like
light).
1. The
laws of physics must be the same for all inertial
observers.
2. The
velocity of light is the same as measured by all
inertial observers.
Although the
Michelson-Morley
results can be considered as proof of the
constancy of the velocity of light, Einstein said that
he had convinced himself of this fact before seeing
the direct experimental evidence.