## ABSTRACT

This chapter provides a recipe for passing from the classical equations of motion to quantum equations. It discusses the relation between observed and calculated quantities. The chapter explains the convenient Dirac notation and a geometric interpretation of quantum mechanics. It deals with an important law of statistical physics known as the fluctuation-dissipation theorem (FDT). It describes the notions of relaxation and the thermostat and derives the simplest kinetic equation, which differs from dynamic equations in that it allows for an interaction with the thermostat. It should be emphasized that, unlike a FDT, the solution of a kinetic equation defines all the moments of a nonequilibrium system, including its susceptibility—the ratio of the first moment to the force. In principle, then, a kinetic equation describes the change of all the statistical properties of a system under the influence of the thermostat and external forces.