 As Rindler’s description of where the rest energy resides makes clear, at each level of analysis, there are two main contributors to the rest energy of the “macroscopic” sample we are analyzing: (1) the energy equivalent of the sum of the rest-masses of the constituent elements considered as fundamental and (2) the sum of the energy “stored” or “carried” by the constituent elements. To perform this kind of analysis for a concrete object is a rather subtle affair. So physicists and philosophers writing about mass-energy equivalence tend to focus on the highly idealized notion of an ideal gas. 