Once the state postulate is given the other unspecified properties must assume certain values.
[1][3] A system is considered to be a simple compressible one in the absence of certain effects which are uncommon in many engineering applications.
For such a system, only two independent intensive variables are sufficient to derive all the others by use of an equation of state.
In the case of a more complex system, additional variables must be measured in order to solve for the complete state.
(e.g., boiling point (temperature) depends on elevation (ambient pressure)).