ABSTRACT
This chapter requires a physical theory which explains the configuration and motion of the system of galaxies, that is, they want a dynamics as well as a kinematics, to make a complete mechanics. At the outset they can neglect all quantum effects, because these are only important for small masses and on a microscopic scale. Mechanics is the study of the motion of a system as it results from interactions between its parts and influences from outside. In Newtonian mechanics, 'interactions' and 'influences' are described by forces. The special frames in which Newtonian mechanics can be employed are called inertial frames. In particular, this implies that in general relativity that must not use Newtonian absolute time together with three space coordinates, but as in special relativity we should specify events in spacetime. The history of a body moving along a path in space is summed up by a line in spacetime.
