 The demerits of either ontological interpretation of mass-energy equivalence are that it rests upon the same-property interpretation of \(E_o = mc^2\). As we have discussed above (see  Section 2.4),  while one can adopt the same-property interpretation, to do so one must make additional assumptions concerning the nature of matter. Furthermore, the ontological interpretation rests on what nowadays seems like a rather crude distinction between “matter” and “fields.” To be sure, mass-energy equivalence has figured prominently in physicists’ conception of matter in no small part because it does open up the door to a description of what we ordinarily regard as ponderable matter in terms of fields, since the energy of the field at one level can manifest itself as mass one level up. However, the inference from mass-energy equivalence to the fundamental ontology of modern physics seems far more subtle than either Enstein and Infeld or Zahar suggest.