Beginning from two simple assumptions, (i) the speed of light is a universal constant, or its
equivalent, the spacetime intervals are Lorentz invariant, and (ii) there are mutually interacting
particles, with a covariant ‘source-field’ equation, one arrives at a class of field equations of
which the standard electromagnetism (EM) and electrodynamics are special cases. The formalism,
depending on how one formulates the source-field equation, allows one to speculate magnetic
monopoles, massive photons, nonlinear EMs, and more.