Objects such as elementary particles interact by so-called fundamental interactions that can be described either by fields or by interaction quanta. Pursuing the equivalence of mass and energy, it is desirable to explain the mass of elementary particles just by the energy of its fields. For this, the contemporary standard model needs additional mechanisms beyond the four well-known fundamental interactions (for instance, like Higgs-Bosons). But up to now such hypothetical mechanisms could not be verified experimentally. Even for the very simple example of the electron, Feynman points out the contradiction between the energy in its electric field and its mass. This article demonstrates that this contradiction can be dissolved as soon as all fields of the particle are taken into account. In the case of the electron, these are mainly the electrostatic and magnetic fields, because the field of weak interaction contains a comparatively small amount of energy and the strong interaction does not work on electrons. The article explains the mass of the electron completely by the energy of its fields.