"Sadi’s pamphlet finds its way into the hands of... Rudolf Clausius. It is he who grasps the fundamental issue at stake, formulating a law that was destined to become famous: if nothing else around it changes, heat cannot pass from a cold body to a hot one. ...[A] ball may fall, but it can also come back up, by rebounding... Heat cannot. This is the only basic law of physics that distinguishes the past from the future. None of the others do. Not Newton's laws governing... mechanics... not the equations for electricity and magnatism... by Maxwell. Not Einstein's on relativistic gravity, nor those of quantum mechanics... by Heisenberg, Schrödinger, and Dirac. Not those for elementary particles... by twentieth-century physicists. Not one of these distinguishes... past from... future. If a sequence of events is allowed by these equations, so is the same sequence run backward in time."