Einstein (later parroted by Feynman https://feynmanlectures.caltech.edu/I_15.html
) used to teach that variable speed of light contradicts the principle of relativity. So the constant speed of light, a blatant nonsense, became a logical consequence of (that is, just as true as) the principle of relativity.
Leonard Susskind found it profitable to formalize the Einstein-Feynman fraudulent argument. Here is perhaps the most idiotic syllogism in the history of science:
Premise 1: The laws of physics are the same in every inertial frame (principle of relativity).
Premise 2: Einstein said that the speed of light is a law of physics.
Conclusion: The speed of light is the same in every inertial frame.
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." https://youtu.be/toGH5BdgRZ4?t=626
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