In regard to why the EM momentum-energy stress tensor is not enough to correctly define light momentum, the study also provides a strong mathematical argument that the momentum conservation equations derived from EM stress-energy tensors are all differential equations, and they can be converted one to the other through Maxwell equations; thus "Maxwell equations support various forms of momentum conservation equations, which is a kind of indeterminacy. However it is this indeterminacy that results in the question of light momentum." To remove the indeterminacy, the study argues, the principle of relativity is indispensable. "This principle is a restriction but also is a guide in formulating physical theories. According to this principle, there is "no preferred" inertial frame for descriptions of physical phenomena. For example, Maxwell equations, global momentum and energy conservation laws, Fermat's principle, and Einstein's light-quantum hypothesis are equally valid in all inertial frames, no matter whether the medium is moving or at rest, and no matter whether the space is fully or partially filled with a medium."