The structure specified by treating space and time together as a four-dimensional manifold. Points in space-time are called events. In the theory of relativity, each event in space-time is associated with a past light cone (the set of past events that could possibly have influenced it) and a future light cone (the set of future events that it could possibly influence), where the possibility in question is limited by the speed of light. Regions outside each light cone of an event are the ‘elsewhere’. D’Alembert may have been the first to suggest that time is regarded as a fourth dimension in his article ‘Dimension’ for the Encyclopédie, but he could not foresee the main physical point of the concept, which is that in relativity theory space and time can be traded, like mass and energy; what is invariant in relating two events is not their spatial distance, nor their temporal distance, but only their distance in space-time.