“What Is The Theory Of Relativity?”

Albert Einstein, “What Is The Theory Of Relativity?” (November 28, 1919)
In the London Times, Einstein explained this fact about his theory:
"The second principle, on which the special theory of relativity rests,
is the "principle of the constant velocity of light in vacuo." This
principle asserts that light in vacuo always has a definite velocity of
propagation (independent of the state of motion of the observer or of
the source of the light). The confidence which physicists place in this
principle springs from the successes achieved by the electrodynamics of
Maxwell and Lorentz."
The velocity of waves does not include the velocity of the source
because compression waves are formed, constraining the speed to that
dictated by the medium.
The velocity of both particles and waves includes that of the observer.
To deny that light velocity involves that of the observer is stupid,
illogical, and irrational nonsense. It is a denial of relative motion
itself.