 In fact, the ensemble will not evolve if it contains all past and future phases of the system.Such a statistical ensemble, one that does not change over time, is called "stationary" and can be said to be in "statistical equilibrium".The study of thermodynamics is concerned with systems that appear to human perception to be "static" (despite the motion of their internal parts), and which can be described simply by a set of macroscopically observable variables.