 Bondi and Spurgin’s (1987) interpretation of mass-energy equivalence has been influential especially among physicists concerned with physics education. In an article where they complained about how students often misunderstand Einstein’s famous equation, Bondi and Spurgin argued that Einstein’s equation does not entail that mass and energy are the same property any more than the equation \(m = \varrho V\) (where \(m\) is mass, \(V\) is volume, and \(\varrho\) is density) entails that mass and volume are the same. Just as in the case of mass and volume, Bondi and Spurgin argue, mass and energy have different dimensions. Ultimately, this reduces to a disagreement with philosophers such as Torretti who would argue that time, as a dimension, is no different than any one of the spatial dimensions. Note well that this is not an issue about the units we use for measuring mass (or energy).