 Einstein and Infeld (1938) and Zahar (1989) have both argued that \(E_o = mc^2\) has ontological consequences. Both of the Einstein-Infeld and Zahar interpretations begin by adopting the same-property interpretation of \(E_o = mc^2\). Thus, according to both interpretations, mass and energy are the same properties of physical systems. Furthermore, both the Einstein-Infeld and Zahar interpretations use a rudimentary distinction between “matter” and “fields.” According to this somewhat dated distinction, classical physics includes two fundamental substances: matter, by which one means ponderable material stuff, and fields, by which one means physical fields such as the electromagnetic field. For both Einstein and Infeld and Zahar, matter and fields in classical physics are distinguished by the properties they bear. Matter has both mass and energy, whereas fields only have energy. However, since the equivalence of mass and energy entails that mass and energy are really the same physical property after all, say Einstein and Infeld and Zahar, one can no longer distinguish between matter and fields, as both now have both mass and energy.