It's a well-established fact that a given ray of light propagates in vacuum at the constant speed c ≈ 300'000'000 m/s, relative to every inertial frame. 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.
Note: throughout this essay, light is always meant to travel in vacuum.
To eliminate the contradiction, it seems necessary that our intuitive concepts about space and time are revisited and challenged. 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.