If it were true that non-inertial motion, such as spin, is required for magnetism to exist, then one could say that a lack of non-inertial motion implies lack of magnetism. However, whether a separate body is traveling inertially or non-inertially in relation to spacetime is not something dependent on the observer, and therefore, the notion that magnetism is fully dependent on the existence of non-inertial motion conflicts with the idea that electric fields and magnetism are completely interchangeable according to the frame of reference. Additionally, I understand that quantum particles possess intrinsic spin magnetic moments, which are not understood as classical spins. The observed magnetic moment that results from these "intrinsic" spins are not subject to variations resulting from relative motions of the observer. In other words, they appear to be defined on the basis of some "intrinsic" non-inertial motion, despite the fact that classical theory is no longer accepted as an explanation for this spin. Given the invariance with respect to an observer as to whether a distant object is following a spacetime geodesic or not, as well as the invariance with respect to an observer as to whether an object is spinning or not (i.e. a rotating frame), it would seem that magnetism, which is usually regarded as a relative phenomenon, may have an underlying full dependence on there being non-inertial motion.