In I, the forces acting on an arbitrary portion of an electrically or magnetically polarized body are calculated without introduction of energy arguments, of an atomic model of the polarization, or of assumptions in regard to the physical significance of the macroscopic field vectors within polarized matter. It is pointed out that a magneto-mechanical formula derived by use of ampèrian currents and one derived by use of magnetic poles will, in general, require different “stress” systems in a given physical situation, because a part of the force that is included in the long-range force in one calculation is attributed to stresses in the other. In the present treatment, dipole-dipole interactions are taken as fundamental; poles and ampèrian currents are regarded as purely formal devices; and the two are treated on an equal footing, so that two equivalent sets of formulas are obtained. In II, the general formulas of I are specialized to the case of a fluid. Formulas relating to electrostriction and to the effect of immersing a body in a fluid are derived; their relation to the traditional formulas (based on energy arguments) is discussed.