The Argument From Free Will (AFFW), is traditionally considered an argument against the existence of an omniscient God. More appropriately, it is an argument for the incompatibility between the full exercise by God of His Omniscience , and genuine human free will.
The AFFW is closely related to the issue of Theological Fatalism, which is about whether God's foreknowledge of human actions imply that humans have no freewill Moses Maimonides formulated the argument, in the traditional manner, in terms of good and evil actions, as follows:
… "Does God know or does He not know that a certain individual will be good or bad? If thou sayest 'He knows', then it necessarily follows that [that] man is compelled to act as God knew beforehand he would act, otherwise God's knowledge would be imperfect.…"|30px|30px
More in general, the Argument From Free Will (AFFW) can be applied to all free-will actions by individuals. In modern terms, the argument if formulated typically as follows
If omniscient is used in the first sense then the argument's applicability depends on what the god in question chooses to know, and therefore it is not a complete argument against the existence of God. In both cases the argument depends on the assumption that it is logically possible for God to know every choice that he will make in advance of making that choice.
One criticism of the Argument from Free Will is that in point 4 of the proof it simply assumes that foreknowledge and free will are incompatible. It uses circular logic to "prove" this, by simply stating that "a being that knows its choices in advance has no potential to avoid its choices". Point 4 is therefore saying, in essence, "A being that knows its choices in advance has no free will, and therefore has no free will". By assuming what it is trying to prove, that point undermines the entire argument.
Specifically, point 4 commits the modal fallacy of assuming that because some choice is known to be true, it must be necessarily true (i.e. there is no way it could possibly be false). This fallacy is most easily demonstrated by an example that has nothing to do with omniscience: While it is true that a person has one brother and one sister, it is not necessarily true. For example, it is possible that that person could have had two brothers, or two sisters, or no siblings at all. Logically, the truth value of some proposition can not be used to infer that the same proposition is necessarily true.
Using logical terminology and applying it to AFFW, there is a marked distinction between the statement “It is impossible (for God to know a future action to be true and for that action to not occur)” and the statement “If God knows that a future action is true, then it is impossible for that action to not occur.” While the two statements may seem to say the same thing, they are not logically equivalent. The second sentence is false because it commits the modal fallacy of saying that a certain action is impossible, instead of saying that the two propositions (God knows a future action to be true, and that action does not occur) are jointly impossible. Simply asserting that God knows a future action does not make it impossible for that action not to occur. The confusion comes in mistaking a semantic relation between two events for a causal relation between two events.
With these assumptions more explicitly stated, the proof becomes:
This claims to prove that at time T, person X is unable to do any action other than A. However, you could also remove steps 4–6, and arrive at the same conclusion. This is called logical determinism, and it suffers from the same modal fallacy as AFFW. If a certain proposition is true, that does not imply that the proposition is logically necessary. Once you remove the invalid assertion, then the argument for logical determinism is shown to be false. Similarly, when that same invalid assertion is removed from AFFW (“by the definitions of ‘knowledge’ and ‘choice’, if one knows for certain what choice one will make in the future, one will not be able to make the opposite choice”), the proof is shown to be false. Another common criticism of AFFW argues that the apparent contradiction arises from an attempt to attribute temporal attributes to an atemporal idea or being. In this view, God exists beyond the constraints of linear time, and the temporal terminology used by AFFW is meaningless when applied to him: God doesn't need to know any event "before" it happens but rather is capable of knowing/experiencing it "while" it happens, since God’s knowledge extends beyond linear time. In that case, God would appear (from a temporal perspective) to know an event before it happens, as stated in AFFW. AFFW proponents respond that the above argument does not change the contradiction. They claim that omniscience by definition means that God has knowledge of all human events of all human times, even if God's own relationship to time is entirely different from ours.
A counter-argument to the above argument is that God’s knowledge is a result of the free-will agent's choice, not the cause of it, and therefore no contradiction exists (whether God is temporal or atemporal).
One could reply that cause and result are two ends of a relation, and we recognize which is which by their time order (or at least logical sequence, i.e. the prerequisites), since cause is what takes place earlier than the result. However, making this claim ignores the central concept behind omniscience, where the effect (knowledge of an action) specifically and deliberately precedes the cause (the action itself). It would be erroneous to ascribe normal, temporal cause-effect relationships to the case of omniscience.