"So chapter 5 discusses the basic principle of relativity which, as I understand it, is mainly the Galilean idea of relative velocity and direction, pretty straightforward stuff. Einstein talks about reference frame K and another reference frame K' which moves with respect to K. Then in chapter 7, he discusses the propagation of light. As he's building to the point that c is constant regardless of reference frame, he uses the example of light c being measured by an observer riding in a train and he says that the apparent speed of light w, according to Galilei-Newton mechanics, would be the difference between c and the observer's velocity v. I know that this is experimentally incorrect but what I don't get is that he says that that point is "in conflict with the principle of relativity set forth in section 5" but I don't see how that's true. [...] I'm just kind of confused by what he means here because it seems to me as if he's saying the opposite of the truth. It seems that, in Galilei-Newtonian relativity, the idea that w = c - v, though experimentally incorrect, is in direct accord with the principle of relativity..." https://www.reddit.com/r/Physics/com...and_something/

My comment on Reddit (Einsteinians may delete it of course):

Yes, w = c - v is in direct accord with the principle of relativity, and no, it is not "experimentally incorrect". Banesh Hoffmann, Einstein's collaborator, admits that, originally ("without recourse to contracting lengths, local time, or Lorentz transformations"), the Michelson-Morley experiment unambiguously proved Newton's variable speed of light and disproved the constant speed of light:

"Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether." Banesh Hoffmann, Relativity and Its Roots, p.92

Pentcho Valev