Is it that within a certain frame of reference, be it an inertial frame of reference, no experiment can be peformed which would indicate that you are moving? And that because of this, the aether could not exist, otherwise a common train model, where the speed of light would be measured to be less if it was travelling against the aether, and if it were travelling with the aether, the speed would be increased, hence you could tell that you are moving and thus, the two theories cannot coexist.