Information theory described in Part 1 provides a meaning for the concept of entropy independent of the field of thermodynamics. Using this meaning (uncertainty) it is possible to derive all of statistical and classical thermodynamics in a direct and simple way. Many of the concepts and definitions of classical thermodynamics are given a new interpretation. Using information theory as a basis for a statistical description of an open system, in Part 2, the laws and theorems of thermodynamics are seen to follow in a simple way. The coupling of irreversible flows (Onsager’s relation) is seen as a natural connection between thermostatics and thermodynamics if the functions introduced by Massieu are used instead of those introduced by Gibbs.