But you are making a faulty assumption that THAT is the only valid equation to consider. I've given you one other. So which one do you choose? It appears that you simply chose one and ignored the rest of the equations in physics to make your deduction about the "cause" of the speed of light. This is puzzling logic.E and M are the only variables, right? There's no medium that would determine the speed of light, right? So what else could be responsible for the speed of light constant C except some fixed relationship between the two variables?