The electrical resistivity is shown to be expressible in terms of a force-force correlation function which is to be evaluated not for the actual specimen but for a hypothetical system in which the specimen is immersed in an infinite bath of free or Bloch electrons. This generalizes to all scattering strengths a result previously obtained for weak scattering. The generalization does not hold for a time-varying situation. Specializing to an independent-particle framework, the force-force correlation function formula is re-expressible in terms of the total T matrix of the system, defined for the situation in which the specimen acts as a scatterer of an incoming beam of electrons. If one substitutes into the formula the T matrix defined for the isolated specimen, where the waves are scattered into themselves, the resulting expression is identically zero, a result to be expected on general grounds.