  One approach, that of P. Duhem and E. Mach and the “energeticists,” was to insist that these principles were autonomous phenomenological laws that needed no further grounding in some other physical principles. An alternative approach was to claim that the energy in a body stored as heat content was an energy of motion of some kind of hidden, microscopic constituents of the body, and to insist that the laws noted, the thermodynamic principles, needed to be accounted for out of the constitution of the macroscopic object out of its parts and the fundamental dynamical laws governing the motion of those parts. This is the kinetic theory of heat.