“Shoemaker’s world seems possible, but one question ispressing. What causes the end of a freeze? In the case of alocal freeze, say in A, it may be events in B or C that causethe freeze in A to end. But what causes the end of a globalfreeze? When the entire universe is frozen, what could causeit to unfreeze? Not events occurring during the year of globalfreeze, for there are none. Nor can the cause be any eventprior to, or simultaneous with the beginning of, the freeze, forthen the freeze would be over as soon as it had begun.”