 The conservation of energy tells us nothing about temporally asymmetric behavior. It doesn’t follow from the First Law that interacting systems quickly tend to approach equilibrium, and once achieved, never leave this state. It is perfectly consistent with the First Law that systems in equilibrium leave equilibrium. In particular, no limitations are placed on transforming energy from one form into another, so the Law permits the possibility of machines that remove heat from their environment and turn it into work (a so-called perpetual mobile of the second kind). To rule out such machines, and more generally, to capture the amazingly general temporally asymmetric behavior we find, another law is needed. Although Carnot was the first to state it, the formulations of Kelvin and Clausius are standard: