"When he combined the principle of relativity with the constant speed of light, it became clear to Einstein that the speed of light was also independent of the speed of the observer (as well as of the speed of the source of the light), and that everyone in the universe, no matter how fast they were moving, would always measure the speed of light at exactly the same 300,000 km/s." http://www.physicsoftheuniverse.com/topics_relativity_light.html
That is, Einstein postulated that the principle of relativity is correct and that the speed of light is independent of the speed of the source, and deduced nonsense:
John Stachel: "But this seems to be nonsense. How can it happen that the speed of light relative to an observer cannot be increased or decreased if that observer moves towards or away from a light beam? Einstein states that he wrestled with this problem over a lengthy period of time, to the point of despair." http://www.aip.org/history/exhibits/einstein/essay-einstein-relativity.htm
That was a typical reductio ad absurdum argument. Einstein should have concluded that his second postulate - the speed of light is independent of the speed of the source - is false.