Hi all I was recently told that certain postulates of SR are equivalent or one can be derived from the other. Namely 1) The speed of light in vacuum is constant and is independent of the speed of emitting body; 2) The speed of light is impassable limit; I did some search and found quite different statements though. I found that postulate #1 was first postulated by Einstein in his "ON THE ELECTRODYNAMICS OF MOVING BODIES". One article said even that Einstein did not say light speed is impassable limit. Postulate #2 was first proposed by Poincare. My question is: Can #1 be derived from #2 or vice versa? Thank you all.