 Interpretations of \(E_o = mc^2\) that hold that mass and energy are distinct properties of physical systems need not, of course, assume that \(q\) is different from zero. Such interpretations can simply leave the value of \(q\) to be determined empirically, for as we have seen such interpretations argue for treating mass and energy as distinct properties on different grounds. Nevertheless, the Bondi-Spurgin interpretation does seem to adopt implicitly a hypothesis concerning the nature of matter.