I have read different version/interpretations of Einstein derivation of the formula, no one is logically satisfying. But my question is more general: the value of energy in rest mass is a scalar, which is not related to anything, how can we use logic or math to derive such a value? Stretching it a bit to make myself clear, how can you derive the number of fingers in a human being? There is sure anarcane reason, but that is the way Nature decided and has it, how can you prove/derive it if you have no direct acces to it? Because of annihilation we knew that it is in the region of c^2, but many coefficients have been suggested and they are all right or wrong, depending on the init of energy we choose. They choose to adopt that value to honour him, but I read that he himself had suggested the factor of 1/2, and when he was accused of plagiarism, he changed it to 1. If there were a compelling reason or rationale, you do not change it overnight.