Ideas on a source of inertia from fixed stars have crossed Physics least Mach onwards. Equations "Maxwell-like" for gravitation and inertia were obtained by several authors as a subspecies of the simplified theory of General Relativity. A precursor was Dennis Sciama. Introducing a four-potential, I submit here a tentative interpretation of our laws about gravity and inertia, in complete analogy with electromagnetism. In classical mechanics is not introduced, usually, a four potential. The field produced by this four-potential describes both gravitational and inertial forces. Admit the gauge transformation on potential is equivalent to enunciate the equivalence principle, and vice versa. Inertial forces (ex. Coriolis force) are interpreted as a field action. All the inertia is interpreted as a field action. The physical presence of this field seems to be a fact, even more concrete the usual admission of inertial "fictitious forces".