Someone said that entire theory of relativity hinges on the assumption that the speed of light is constant in a one way direction between any two points and that this cannot be proven scientifically; it must be assumed. A response to this was: constancy of the speed of light through a given medium is not an assumption. We assume this is constant in all reference frames because we have no evidence otherwise. Thus, it is empirically demonstrated in a reliable fashion and our models based on this data work. Who is correct? Please elaborate. Thank you.