As WE HAVE SEEN in Chap. 11, the systems of thermodynamics are ordinary objects: solids, liquids and gases with definite boundaries. The observables of interest are somewhat more remote from direct perception than the visual properties on which mechanics concentrates attention. Temperature, pressure, and entropy lack the intuitive immediacy of positions and velocities. They are bound to Nature by more extended and more complex correspondences and lead to concepts that are more abstract. All this makes the problem of explanation in thermodynamics rather unique and interesting and gives it some features which form a bridge with quantum mechanics.
In thermodynamics proper, observables are connected by what are sometimes called empirical relations, with the term empirical understood in a very limited and specific way. The "laws of motion" in this science are primitive equations combining the observables themselves. In a certain sense, the laws do not say "why" bodies behave thermodynamically as they do. While this connotes no defect of the methods of thermodynamics as a science, it nevertheless raises a question as to the possibility of other modes of explanation, of theories that "go behind" the phenomenologic structure of thermodynamics and its minimal assumptions. The question arises primarily in view of the postulate of extensibility (Chap. 5), which predisposes science to seek ways whereby the explanatory repertoire of one theory can be made to serve others as well. Now, clearly, the bodies of thermodynamics are also systems of mechanics, and it is indicated that one should inquire whether the laws of mechanics can produce, or at least simulate, the equations of thermodynamics.