The argument from free will, also called the paradox of free will or theological fatalism, contends that omniscience and free will are incompatible and that any conception of God that incorporates both properties is therefore inconceivable. See the various controversies over claims of God's omniscience, in particular the critical notion of foreknowledge. These arguments are deeply concerned with the implications of predestination.
Some arguments against the existence of God focus on the supposed incoherence of humankind possessing free will and God's omniscience. These arguments are deeply concerned with the implications of predestination.
Moses Maimonides formulated an argument regarding a person's free will, in traditional 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 the man is compelled to act as God knew beforehand how he would act, otherwise, God's knowledge would be imperfect...."
A logical formulation of this argument might go as follows:
Norman Swartz, however, contends that the above arguments commit the modal fallacy. In particular, he asserts that these arguments assume that if C is true, it becomes necessary for C to be true, which is incorrect as C is contingent (see modal logic). Otherwise, one can argue that the future is set already regardless of his actions.
Other means of reconciling God's omniscience with human free will have been proposed. Some have attempted to redefine or reconceptualize free will:
A proposition first offered by Boethius and later by Thomas Aquinas[note 1] and C. S. Lewis, suggests that God's perception of time is different, and that this is relevant to our understanding of our own free will. In his book Mere Christianity, Lewis argues that God is actually outside time and therefore does not "foresee" events, but rather simply observes them all at once. He explains:
But suppose God is outside and above the Time-line. In that case, what we call "tomorrow" is visible to Him in just the same way as what we call "today". All the days are "Now" for Him. He does not remember you doing things yesterday, He simply sees you doing them: because, though you have lost yesterday, He has not. He does not "foresee" you doing things tomorrow, He simply sees you doing them: because, though tomorrow is not yet there for you, it is for Him. You never supposed that your actions at this moment were any less free because God knows what you are doing. Well, He knows your tomorrow's actions in just the same way--because He is already in tomorrow and can simply watch you. In a sense, He does not know your action till you have done it: but then the moment at which you have done it is already "Now" for Him.
A common objection is to argue that Molinism, or the belief that God can know counterfactually the actions of his creations, is true. This has been used as an argument by Alvin Plantinga and William Lane Craig, amongst others.
Dan Barker suggests that this can lead to a "Free will Argument for the Nonexistence of God" on the grounds that God's omniscience is incompatible with God having free will and that if God does not have free will God is not a personal being.
These two terms are known as inherent and total omniscience, respectively.