"’-and if the space terms predominate in the expression for the interval between event-points, the interval is said to be spacelike, since it is then possible to select a co-ordinate system- belonging to an observer with an admissible velocity-in which the events appear simultaneous, and therefore separated only spatially. If, however, the interval is timelike, the events cannot be simultaneous in any co-ordinate system, but there exists a co-ordinate system in which the space terms will vanish entirely, so that the separation between events will be purely temporal, id est, occurring at the same place, but at different times. Now upon examining the extremals of the real interval-‘"