  The standard method for calculating the properties of an energetically isolated system in equilibrium was initiated by Maxwell and Boltzmann and developed by J. Gibbs as the microcanonical ensemble. Here a probability distribution is imposed over the set of microscopic states compatible with the external constraints imposed on the system. Using this probability distribution, average values of specified functions of the microscopic conditions of the gas (phase averages) are calculated. These are identified with the macroscopic conditions. But a number of questions arise: Why this probability distribution? Why average values for macroscopic conditions? How do phase averages related to measured features of the macroscopic system?