# What information is sufficient for describing a thermodynamic system?

For a single-component system, why are the energy, volume, and number of particles sufficient for describing the thermodynamics of the system? Why just three variables and those three variables in particular?

In the book that I am using (Callen, $$\textit{Thermodynamics and an Introduction to Thermostatistics}$$) he postulates that the macroscopic equilibrium state is characterized by the energy, volume, and particle numbers of its components, but what is the reason for this?