The concept of temperature is as fundamental a physical concept as the three fundamental quantities of mechanics - mass, length, and time. Through the study of such practical problems as how to make a highly efficient steam engine, fundamental physical theories emerge, including the concepts of the quantum theory and the two laws of thermodynamics. The second law, with its irreversibility requirement, predicts an inevitable evolution from other forms of energy into heat. It is the second law alone that provides an "arrow" for the concept of time.

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