  Profound studies by S. Carnot of the ability to extract mechanical work out of engines that ran by virtue of the temperature difference between boiler and condenser led to the introduction by R. Clausius of one more important parameter describing a material system, its entropy. How was the existence of this simple set of parameters for describing matter and the lawlike regularities connecting them to be explained? What accounted for the approach to equilibrium and its time asymmetry? That the heat content of a body was a form of energy, convertible to and from mechanical work formed one fundamental principle. The inability of an isolated system to spontaneously move to a more orderly state, to lower its entropy, constituted another. But why were these laws true?