 The second demerit of the Bondi-Spurgin interpretation, which it shares with all other interpretations of mass-energy equivalence that hold that mass and energy are different properties, is that it remains silent about a central feature of physical systems it uses in explaining apparent conversions of mass and energy. In order to explain purported conversions along the lines suggested by Bondi-Spurgin, one must make the familiar assumption that the energy of the constituents of a system, be it potential energy or kinetic energy, “contributes” to the rest-mass of the system. Thus, for example, in the bombardment of \({^7}\Li\) reaction Bondi and Spurgin must explain the rest-mass of the \({^7}\Li\) in the familiar way, as arising from both the sum of the rest-masses of the nucleons, and the mass-equivalents of their energies. However, the Bondi-Spurgin interpretation offers no explanation concerning why the energy of the constituents of a physical system, be it potential energy or kinetic energy, manifest itself as part of the inertial mass of the system as a whole. Of course, one can always reply that even to ask for this type of explanation is to refuse to accept relativistic thinking fully: the potential and kinetic energies of the constituents contributes to the rest energy of the whole, and because of Einstein’s equation, contributes to the rest-mass of the whole.