On the morning after a battle, Mary prays that her husband has not been killed. Is this a coherent plea? It would seem that the matter has already been decided: Her husband is either alive or dead. If he is dead, then in order to grant Mary’s prayer God would have to change the past retroactively.

“If one does not think of [such a] case, the idea of doing something in order that something else should previously have happened may seem sheer raving insanity,” writes Michael Dummett. “But suppose I hear on the radio that a ship has gone down in the Atlantic two hours previously, and that there were few survivors: my son was on that ship, and I at once utter a prayer that he should have been among the survivors, that he should not have drowned; this is the most natural thing in the world.”

Perhaps God can grant Mary’s prayer without changing the past: Perhaps, using divine foreknowledge, he interceded at the time of the battle knowing that she would later pray for this. “One of the things taken into account in deciding [the outcome], and therefore one of the things that really causes it to happen, may be this very prayer that we are now offering,” writes C.S. Lewis.

But this entails an oddity of its own — such favors, it seems, are available only to those who are in some doubt about a past event. God will intercede today for a prayer tomorrow — but only an uncertain person would make such a prayer. “I may pray that the announcer has made a mistake in not including my son’s name on the list of survivors,” Dummett writes, “but once I am convinced that no mistake has been made, I will not go on praying for him to have survived. I should regard this kind of prayer as something to which it was possible to have recourse only when an ordinary doubt about what had happened could be entertained.”

(Michael Dummett, “Bringing about the Past,” Philosophical Review, July 1964.)