 The lack of a privileged spatial frame, combined with the obvious existence of privileged states of motion—paths defined as rectilinear in space and uniform with respect to time—suggests that the geometrical situation ought to be regarded from a four-dimensional spatio-temporal point of view. The structure defined by the class of inertial frames can be captured in the statement that space-time is a four-dimensional affine space, whose straight lines (geodesics) are the trajectories of particles in uniform rectilinear motion. See Figure 4.