 After the development of relativity theory (a topic that we address in the companion article), and its interpretation as a spacetime theory, it was realized that the notion of spacetime had applicability to a range of theories of mechanics, classical as well as relativistic. In particular, there is a spacetime geometry – ‘Galilean’ or ‘neo-Newtonian’ spacetime (the terms are interchangeable) – for Newtonian mechanics that solves the problem of absolute velocity; an idea exploited by a number of philosophers from the late 1960s onwards (e.g., Stein 1968, Earman 1970, Sklar 1974, and Friedman 1983). For details the reader is referred to the entry on  spacetime: inertial frames,  but the general idea is that although a spatial distance is well-defined between any two simultaneous points of this spacetime, only the temporal interval is well-defined between non-simultaneous points. Thus things are rather unlike Newton’s absolute space, whose points persist through time and maintain their distances: in absolute space the distance between p-now and q-then (where p and q are points) is just the distance between p-now and q-now. However, Galilean spacetime has an ‘affine connection’ which effectively specifies for every point of every continuous curve, the rate at which the curve is changing from straightness at that point; for instance, the straight lines are picked out as those curves whose rate of change from straightness is zero at every  point.[9]