Thermodynamics is a “phenomenal” science. That means that its variables range over macroscopic parameters such as temperature, pressure and volume. These are properties that hold at equilibrium, i.e., when the values of the macroscopic variables remain approximately stable. Whether the microphysics underlying these variables are motive atoms in the void or an imponderable fluid is largely irrelevant to this science. The developers of the theory both prided themselves on this fact and at the same time worried about it. Clausius, for instance, was one of the first to speculate that heat consisted solely of the motion of particles (without an ether), for it made the equivalence of heat with mechanical work less surprising. However, as was common, he kept his “ontological” beliefs separate from his official statement of the principles of thermodynamics because he didn’t wish to (in his words) “taint” the latter with the speculative character of the former.