I'm concerned about your book. I got up to this part: And I got very concerned. Although what you say is true, the fact that you put the last part of the quote in a box, like you did for what you called the Fundamental Principle and its restated version, gave me the impression that you believe that part of the quote is his second postulate. But you are specifically stating that there really is only one principle or postulate and this misrepresents not only what Einstein said but also the fundamental basis for Special Relativity. The actual statement of his second postulate from the introduction of his 1905 paper is: Or later at the beginning of section 2: When he says the "definite velocity c" or the "determined velocity c", he is referring to his comment at the end of the first section where he defines c to be the measured two-way speed of light. But his second postulate is about the unmeasurable one-way speed of light which cannot be part of the first postulate. That's what he means by "propagated" or "ray". He's defining the one-way speed of light to be identical to the two-way speed of light in any system of co-ordinates. This is my concern about your book. Do you eventually clarify this issue?