The Three famous Postulates of Thermodynamics.
- There exists a function called the entropy of extensive parameters of any system, defined for all stabilized states having the property that the entropy maximizes.
- The entropy of a composite system is additive over constituent subsystems. As a function, it's also continuous, differentiable, and monotically increasing with respect to energy.
- If (∂u/∂S)V,N=0, then S=0
The only other thing of note is notice in Postulate Two:
(The Entropy is a) monotonically increasing with respect to energy.
in other words:
∂S/∂U > 0
1/T > 0
Temperature can never be negative.