 Lange seems to use a familiar argument from the Lorentz invariance of certain physical quantities to their “reality.” For Lange, if a physical quantity is not Lorentz invariant, then it is not real in the sense that it does not represent “the objective facts, on which all inertial frames agree” (2002, p. 209). Thus Lange uses Lorentz invariance as a necessary condition for the reality of a physical quantity. However, in several other places, for example when Lange argues for the reality of the Minkowski interval (2002, p. 219) or when he argues for the reality of rest-mass (2002, p. 223), Lange implicitly uses Lorentz invariance as a sufficient condition for the reality of a physical quantity. However, if Lange adopts Lorentz-invariance as both a necessary and sufficient condition for the reality of a physical quantity, then he is committed to the view that rest energy is real for the very same reasons he is committed to the view that rest-mass is real. Thus, Lange’s original suggestion that there can be no physical process of conversion between mass and energy because they have different ontological status seems challenged.