 If we assume that a system is governed by deterministic physical laws, then the laws allow us to derive the occurrence of some event E from appropriate initial and boundary conditions. What combination of initial and boundary conditions are required depends on the type of mathematical equation that express the laws. In the case of hyperbolic equations, such as the wave equation, we can formulate a pure initial value problem. In this case, the event E occurring at spatial location x and time \(t_1\) is derivable from an earlier state S specified on a complete initial value surface in a region surrounding x at time \(t_0.\) What is important is that nothing less than a complete specification of the state on the initial value surface will allow E to be derived from the laws—the laws are silent on how a system with incompletely specified initial conditions may evolve. That is, (a) we need to specify the initial state in the relevant region completely and in whatever detail the laws at issue require; and (b) the relevant region comprising the initial value surface may be quite large. In the case of relativistic theories initial value surfaces consist of entire cross sections of the backward lightcone of E. (The backward lightcone consists of those events from which a signal traveling at most at the speed of light could reach E.) In the case of classical Newtonian theories, which allow for signals propagating at arbitrarily high speeds, S would have to amount to the state of the entire universe at some time \(t_0\) prior to E.