It is a well-established experimental fact that:
(Law) The tip of a ray of light propagates in vacuum at a constant c ≈ 300'000'000 m/s speed, always along a straight line, relative to every inertial frame.
(Corollary) The speed of light in an inertial frame is independent of the state of motion of the entity that emits it.
This is contradictory, since according to classical mechanics the speed of a given entity, i.e. that of the tip of the ray of light in this case, should depend on the reference frame in which it is observed. To eliminate the contradiction, it was necessary in special relativity to revisit and challenge our intuitive concepts about space and time.
In order to ensure that we are not misled by intuition, Einstein suggested that all definitions in physics must be based on measurements which are, in principle, feasible.