The problem of omniscience is usually formulated about whether human being’s having free will is compatible with God’s foreknowledge of our actions. The basic problem is that God know what we will do before we do it which means that it must be true that we perform this action before we actually perform it (knowledge is factive). But if it is already true then how could I do otherwise? If I do in fact have freedom and do otherwise then it seems that I have the power to cause one of God’s beliefs to be false; but that is impossible. God is essentially omniscient and so only has true beliefs. Plantinga famously responded that we can solve this problem by thinking about possible world semantics. Let us suppose that in the actual world I freely choose to drink a Lemon iced tea instead of a peach iced tea on Friday December 11th 2009 at 1145 a.m. Let’s call this T2. Given God’s omniscience then we must suppose that He knew that would perform this action even before my birth. Let’s just pick a date, say October 31st 2008, and call this T1. Then God knows at T1 that I will choose lemon tea at T2. Now the reason Plantinga sees no problem here is because there is a possible world where I freely chose to have the peach tea (call it W2) and in that possible world God knows that I will choose peach tea at T2. So the basic idea is that had I chosen differently God would have a different belief and so we could say that the true belief that God does hold in the actual world would be false in W2 but that doesn’t mean that God has a false belief in the actual world. So, I am free (i.e. there is a possible world where I do otherwise and so it is not necessary that I have lemon tea) and God is still omniscient. Pike’s response to this argument is basically to complain about Plantinga’s analysis of freedom. The question is not whether or not there is some possible world or other where I do otherwise and God knows that I do otherwise. The question is whether or not given the actual world as it is, is there a possible world with exactly the same history as the actual world in which I do otherwise? If there is then God has a false belief in that world because in that world God believes at T1 that I will have lemon tea at T2 but we have just said that in this world I have peach tea at T2. On the other hand if there is no such possible world then it was not really in my power to do otherwise after all. To appreciate the point that Pike is making here we can point out that even the determinist can admit that there is a possible world where I “chose” to have peach tea at T2. It is, of course, not in the subset of possible worlds that have the same history as our world (or our universe for that matter) but surely we can conceive of different subsets with different histories (e.g. possible worlds where the initial force of the big bang is different or in which there is an extra molecule, ungrounded in Kripke’s sense, that effects the outcome of the universe’s history) and so there merely being some other possible world where I have peach tea instead of lemon tea at T2 cannot be what we mean when we say that I am free. We must mean that there is a possible world that is near enough in the space of possible worlds to the actual world such that I could bring it about. And as we have seen it is not obvious that this is possible since in all the possible worlds that are near enough God knows that I will have lemon tea. I find this response very convincing (and I now think that my earlier attempts at this were groping in this direction)
So much then for the traditional problem of omniscience. However, it occurred to me recently that there is, besides this traditional problem, a further problem which we might call ‘the logical problem of omniscience’ on analogy with the logical problem of evil. The logical problem of omniscience suggests that there is a contradiction in the claim that God is all-knowing, all-powerful, morally perfect and has free will. This is a more pressing problem because it threatens to show that belief in an omniscient God with free will is itself an irrational belief. The contradiction arises because included in God’s foreknowledge is knowledge of what His own choices and actions will be. So if God chooses to destroy Sodom at noon on Wednesday 1400 B.C (*Note: This is a made up arbitrary date!*) then it must have been the case the He knew that He would so choose and so knew that He would destroy Sodom at that time on that date. But if God knew this at T0 (say before he created Adam) then how could He have chosen differently in 1400 B.C.? But then either God is not omniscient or he does not have free will neither of which is acceptable.