The argument from foreknowledge is an argument that divine omniscience, or more specifically divine foreknowledge, is inconsistent with human freedom. The argument begins with a consideration of the consequences of God knowing everything. If God knows everything then he knows every act that each of us is going to perform in the future.

If God knows every act that each of us is going to perform in the future, though, then it is not possible for any of us not to perform those acts. For if it were possible for any of us not to perform those acts then it would be possible for us to bring it about that that which God knows is false. Knowledge, of course, by definition, is knowledge of the truth; one cannot know that which is false. The idea that that which God knows could be false is therefore absurd.

Because God’s omniscience entails knowledge of all of our future acts, therefore, it also entails that it is impossible for any of us not to perform those acts.

The argument continues with a consideration of freedom. Freedom, it seems, consists precisely in the ability not to do that which we do, in there being a plurality of acts each of which it is possible for us to choose to perform. If one does not have this ability to choose, i.e. if there is no plurality of acts that it is possible for one to choose to perform, then one cannot be free. If God’s omniscience entails that it is impossible for any of us not to perform those acts that we are going to perform, therefore, then it also entails that none of those acts will be free.

For those that believe that human beings can and do perform acts freely and will continue to do so, the argument from foreknowledge can easily be pressed into service as an argument against the existence of God. For if the existence of an omniscient god is inconsistent with any of our future acts being free, as the argument from foreknowledge appears to demonstrate, then the existence of one free future act entails the non-existence of an omniscient god. Omniscience, though, is a part of the Christian conception of God. If no omniscient god exists, then God does not exist.

The argument from foreknowledge, presented as an argument against the existence of God, may therefore be formalised as follows:

The Argument from Foreknowledge

(1) A necessary condition for an act’s being free is that it is possible for the agent that is going to perform the act not to perform it.
(2) If God knows that an agent is going to perform an act, then it is not possible that the agent is not going to perform it.
Therefore:
(3) If God knows that an agent is going to perform an act, then it is not the case that that act is free.
(4) If an omniscient God exists, then if an agent is going to perform an act then God knows that that agent is going to perform that act.
Therefore:
(5) If an omniscient God exists, then if an agent is going to perform an act then it is not the case that that agent is going to perform that act freely.
(6) There is an agent that is going to perform an act freely.
Therefore:
(7) It is not the case that an omniscient God exists.

One possible concern with the argument from foreknowledge is that it appears to equivocate between different senses of “possible”. The “possible” in (1), arguably, does not have the same meaning as the “possible” in (2). The “possible” in (1) appears to mean “possible given everything that is logically prior to the agent’s decision to perform the act”. The “possible” in (2) appears to mean “possible given everything that is logically prior to God’s knowledge that the agent will perform the act”. If there is some significant difference between the senses of “possible” used in (1) and (2), of course, then the argument will fail. A demonstration that God’s knowledge that the agent will perform the act entails that it is impossible in one sense—the sense in (1)—that the agents will not perform the act is not a demonstration that it is impossible in some other sense—the sense in (2)—that the agent will not perform the act. If the argument from foreknowledge equivocates on senses of “possible” in this way, then, then (3) will not follow from (1) and (2).

An alternative response to the argument from foreknowledge invokes the argument from future facts. The argument from foreknowledge purports to demonstrate that divine omniscience is inconsistent with future human freedom. What appears to be inconsistent with future human freedom, though, is not the existence of a being that knows facts about the future, but the existence of those facts about the future whether they are known by any being or not. The argument from foreknowledge can, it seems, be stripped of all references to God knowing the future without losing any of its force. What remains after this process is an argument that there are no truths about the future, a counter-intuitive conclusion. If this argument is unsound, if there are facts about the future, then the argument from foreknowledge must also be unsound. If this argument is sound, if there are no facts about the future, though, then God’s knowing all facts would not threaten human freedom; premise (4) of the argument from foreknowledge would be false. Either way, then, the argument from foreknowledge fails.