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Postulate 1: All inertial frames are equivalent
Postulate 2: The speed of light in vacuum has the same value $c$ in any inertial frame.
It can be shown that the Lorentz transformations directly follow from the fist postulate ( and by assuming the universe is isotropic, but this assumption is also needed with the standard derivation). This proof would lead to several cases to consider (namely, $c= \infty$, $0< c^2 < \infty $, $c^2=0$ and $c^2 <0$) however these can all be ruled out by experiment so that we finally reduce to the familiar Lorentz transformations. Why then do we still consider the invariance of the speed of light as a postulate, if it can be inferred from the first postulate?