What is the Second Law?

The thesis of this article is that thermodynamics is a rigorous science, and that the first law and the second law can be stated in an unambiguous and general way so that their implications are concrete and valid for both equilibrium and nonequilibrium states. In this light, we summarize the principles of thermodynamics, and introduce a graphical tool, the energy versus entropy diagram, that is very helpful to explain and grasp the general implications of these principles, especially in the nonequilibrium domain.