 A different puzzle concerning the effectiveness of petitionary prayer arises in connection with divine omniscience, the idea that God knows everything that can be known. If God already knows the future, for instance, then how can petitionary prayer make a difference? The future, after all, is just the set of things that will happen. If God knows the future in all of its detail, then it seems that there is no room for petitionary prayers to be effective: either the thing requested in prayer is something that God already knows will be done, or it isn’t, and either way, it looks like the prayer can make no difference. Like many other questions in theology, this puzzle raises an interesting question about the limits of God’s knowledge. Is it possible for anyone, including God, to know the future in all of its detail? Philosophers disagree sharply about this. Here we will explore briefly three possible answers to this question. (For more on this, see Borland 2006 (Other Internet Resources) and the entries on  omniscience  and  prophecy.)