The Second Postulate (2)
In section 7 of the first part of “Relativity: The Special and General Theory”, Albert Einstein explained it like this:
Suppose one beam of light is sent out from the railway embankment, in the same direction where the train is moving.
To an observer on the embankment, the speed of this beam is c. To an observer on the train, the speed of this beam is c-v, where v is the speed of the train.
But according to the principle of relativity, the speed of light should be the same, whether the reference frame is the train or the embankment. Thus we get an incompatibility issue.
Note: The principle of relativity is how Einstein calls the first postulate.
So, to Einstein, the first postulate leads to this conclusion:
The speed of light is the same in all reference frames.
By now we know that according to Einstein, the "stationary" or "moving" has no real meaning for light.