Statistical mechanics has the goal to calculate the macroscopic properties of systems with a lot of particles based on the microscopic structure of matter. The central point is the concept of the statistical ensemble: instead of describing the actual system one considers a large number of copies of this system, which all may be in different states, but fulfill the macroscopic conditions (e.g. constant total energy and/or particle number for a closed system). A statistical ensemble then is characterized by the probabilities, with which the individual states (often also called micro-states) are represented in the whole ensemble, that is characterized by the macroscopic conditions. Macroscopic properties, that imply a space-time average of an associated observable, then are understood as average values over the statistical ensemble. The concept of the statistical ensemble–together with the classical equations of motion–provides the framework for classical statistical mechanics.