In order for a god to be all-knowing, he must know even the fact of his own omniscience. But can he do this? He may know the totality of facts constituting the world; call this Y. But in order to know that he has mastered Y, he must also know that “There are no facts unknown to me” — and this is beyond Y.

It seems impossible that a god (or anyone) could ever be sure that nothing exists beyond his ken. “It makes no sense to imagine [a god] arriving at this limit, peering beyond it (at what?), and satisfying himself no further facts exist,” writes philosopher Roland Puccetti. But without this certainty he cannot be sure of his own omniscience, and so does not know everything.

A theist might argue that his god has created all the facts in existence. But an omniscient god would have to be sure of even this — that he is the sole creator, and that there are no facts unknown to him. And how could he come to this knowledge?

(Roland Puccetti, “Is Omniscience Possible?”, Australasian Journal of Philosophy, 1963)

An omnipotent god can create a being whose acts are known only to itself.

An omniscient god cannot do this.

It would appear, then, that no god can be both omnipotent and omniscient.

(From Richard R. La Croix.)

If God makes decisions, then he has a future.

But if he’s omniscient, then he already knows that future.

Can he then have free will?

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.)