 The same-property interpretation of mass-energy equivalence rests squarely on the assumption that \(q = 0\). Mass and energy cannot be the same property if there exists matter that has mass some of which cannot ever, under any conditions, be “converted” into energy. However, one could argue that although the same-property interpretation makes this assumption, it is not an unjustified assumption. Currently, physicists do not have any evidence that there exists matter for which \(q\) is not equal to zero. Nevertheless, it seems important, from a philosophical point of view, to recognize that the same-property interpretation depends not only on what one can derive from the postulates of special relativity, but also on evidence from “outside” this theory.