 Conversely, Einstein also emphasized reading his famous equation “in the other direction,” as it were. For example, in his review article from 1907, he says, “with respect to inertia, a mass \(\mu\) is equivalent to an energy content of magnitude \(\mu c^2\)” (Einstein 1907b, p. 287). Einstein uses the phrase “energy content” (translated from the original German “Energieinhalt”) to convey a notion that is new in special relativity: A physical object contains within it energy that, at least in principle, could be transformed into other forms of energy such as kinetic energy (as we now know only too well). Because this rest energy is contained within the boundaries of a body when we treat the object as a whole, physicists, such as Rindler, also call it “internal energy” (Rindler 1991, p. 71). 