 As it happens, Lange’s overall position is not seriously challenged by the ontological status of rest energy. Lange could easily grant that rest energy is a real property of physical systems and still argue (i) that there is no such thing as a physical process of conversion between mass and energy and (ii) that purported conversions result from shifting levels of analysis when we examine a physical system. It is his observations concerning (ii) that force us to face again the question of why the energy of the constituents of a physical system manifests itself as the mass of the system, though admittedly the question itself may simply reveal a failure fully to appreciate a relativistic description of composite systems. 