 Although it is far less common today, one still sometimes hears of Einstein’s equation entailing that matter can be converted into energy. Strictly speaking, this constitutes an elementary category mistake. In relativistic physics, as in classical physics, mass and energy are both regarded as properties of physical systems or properties of the constituents of physical systems. If one wishes to talk about the physical stuff that is the bearer of such properties, then one typically talks about either “matter” or “fields.” The distinction between “matter” and “fields” in modern physics is itself rather subtle in no small part because of the equivalence of mass and energy. Philosophically, to think of fields as stuff is also controversial. 