Four Laws

Thermodynamics is a set of four laws (two of which are really more important) that limit the ways the system can change. It is useful to distinguish state functions (that are materials properties that you would need to measure) for thermodynamic restrictions (which derive from physical laws of nature).

Question: What is the difference between a physical law and a theorem?

The power of thermodynamics is that everything--everything--follows from these laws. How it follows from the laws is the hard part.