Einstein's 1905 first postulate, the principle of relativity, is true, but his second postulate, the constancy of the speed of light, is obviously false. In order to camouflage this obvious falsehood, Einstein deduced the second postulate from the first one, relying on the following logical principle:
If the premise (principle of relativity) is true, and if the consequence (constancy of the speed of light) is validly deduced from the premise, then the consequence is true as well.
The crucial word is "validly". Einstein's deduction was invalid but in physics logical invalidity is difficult to detect. Here is the deduction:
The speed of light is constant, c, because, if it is variable ("comes out smaller than c"), then "this result comes into conflict with the principle of relativity".
Albert Einstein: "If a ray of light be sent along the embankment, we see from the above that the tip of the ray will be transmitted with the velocity c relative to the embankment. Now let us suppose that our railway carriage is again travelling along the railway lines with the velocity v, and that its direction is the same as that of the ray of light, but its velocity of course much less. Let us inquire about the velocity of propagation of the ray of light relative to the carriage. It is obvious that we can here apply the consideration of the previous section, since the ray of light plays the part of the man walking along relatively to the carriage. The velocity W of the man relative to the embankment is here replaced by the velocity of light relative to the embankment. w is the required velocity of light with respect to the carriage, and we have w = c - v. The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c. But this result comes into conflict with the principle of relativity set forth in Section 5." http://www.bartleby.com/173/7.html
Richard Feynman parroted Einstein's invalid deduction (exactly the same fraudulent assertion: the principle of relativity prevents the speed of light from being variable, c'=c-u):
Richard Feynman: "The principle of relativity was first stated by Newton, in one of his corollaries to the laws of motion: “The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line.” This means, for example, that if a space ship is drifting along at a uniform speed, all experiments performed in the space ship and all the phenomena in the space ship will appear the same as if the ship were not moving, provided, of course, that one does not look outside...Suppose we are riding in a car that is going at a speed u, and light from the rear is going past the car with speed c...according to the Galilean transformation the apparent speed of the passing light, as we measure it in the car, should not be c but should be c-u. For instance, if the car is going 100,000 mi/sec, and the light is going 186,000 mi/sec, then apparently the light going past the car should go 86,000 mi/sec. In any case, by measuring the speed of the light going past the car (if the Galilean transformation is correct for light), one could determine the speed of the car. A number of experiments based on this general idea were performed to determine the velocity of the earth, but they all failed - they gave no velocity at all." https://www.feynmanlectures.caltech.edu/I_15.html
Today's Einsteinians diligently teach Einstein's invalid deduction. The constancy of the speed of light follows from the principle of relativity, they proclaim, and that's it:
Dave Slaven: "Einstein's first postulate seems perfectly reasonable. And his second postulate follows very reasonably from his first. How strange that the consequences will seem so unreasonable." http://webs.morningside.edu/slaven/Physics/relativity/relativity3.html
Professor Raymond Flood: "A consequence of Einstein's principle of relativity is that the speed of light in vacuum has the same value in two uniformly moving frames of reference." View: https://youtu.be/IjRSYv7u3T4?t=304
Chad Orzel: "The core idea of Einstein's theory of relativity can fit on a bumper sticker: The Laws Of Physics Do Not Depend On How You're Moving. Absolutely everything else follows from the simple realization that physics must appear exactly the same to person in motion as to a person at rest - the constant speed of light, the slowing of time for moving observers, E=mc2, black holes, even the expanding universe (I've written a whole book about this, explained through imaginary conversations with my dog)." http://www.forbes.com/sites/chadorzel/2015/05/29/four-reasons-to-not-fear-physics/
Michael Fowler: "Therefore, demanding that the laws of physics are the same in all inertial frames implies that the speed of any light wave, measured in any inertial frame, must be 186,300 miles per second. This then is the entire content of the Theory of Special Relativity: the Laws of Physics are the same in any inertial frame, and, in particular, any measurement of the speed of light in any inertial frame will always give 186,300 miles per second." http://galileo.phys.virginia.edu/classes/109/lectures/spec_rel.html
Leonard Susskind: "The principle of relativity is that the laws of physics are the same in every reference frame. That principle existed before Einstein. Einstein added one law of physics - the law of physics is that the speed of light is the speed of light, c. If you combine the two things together - that the laws of physics are the same in every reference frame, and that it's a law of physics that light moves with certain velocity, you come to the conclusion that light must move with the same velocity in every reference frame. Why? Because the principle of relativity says that the laws of physics are the same in every reference frame, and Einstein announced that it is a law of physics that light moves with a certain velocity." View: https://youtu.be/toGH5BdgRZ4?t=626