Invariably, in my logic and philosophy of religion classes, a student would ask this question: If God knows what I am going to do, then it seems I have no choice but to do it? But that does not follow, logically. God's foreknowledge is not incompatible with libertarian free will. To think so is to have made a mistake in modal logic.
In answering them I would take a marker and begin to explain, using the dry erase board.
Here G = God knows John will eat an orange tonight.
O = John will eat an orange tonight.
~ = 'not.'
◊ = 'possible'
⊃ = 'If..., then'
• = 'and'
The following statement is True: ~◊(G • ~O) [It is not possible that God knows John will eat an orange tonight and John not eat an orange tonight.]
The following statement is False (this is a modal fallacy): G⊃ ~◊~O [If God knows John will eat an orange tonight than it is not possible that John not eat an orange tonight. Or: If God knows John will eat an orange tonight than it is logically necessary that John eat an orange tonight. There is nothing logically necessary about John eats an orange tonight.]
Here's some explanation.
THE COMPATIBILITY OF DIVINE FOREKNOWLEDGE AND HUMAN FREEDOM
1. Some say, “If God knows what choices people are going to make, then we do not have free will in making those choices.”
2. Plantinga (and others) show that this reasoning is not logical. It commits a fallacy in modal logic.
3. Modal logic concerns the different “modes” of the verb “to be.”
a. The 3 modes of “to be” are:
i. Possibility
ii. Probability
iii. Necessity
b. Consider the statement: The coffee in this cup is hot. On modal logic, is this statement true?
i. Possibly? Yes.
ii. Probably? Yes (more or less).
iii. Necessarily? No.
c. If the statement The coffee in this cup is hot were necessarily true, then the coffee in the cup could not not be hot. But that is impossible.
d. No contingent statement can be necessarily true. To claim that is commit a modal fallacy.
4. Consider the statement John will eat an orange tonight. (Call this J)
a. Is this possible? Yes.
b. Is this probable? Yes (more or less).
c. Is this necessarily true? No. Because if it were, then John could not not eat an orange tonight.
d. What if the statement is false. Is it then necessarily false? No.
5. Let G mean: God knows John will eat an orange tonight.
6. Call this Statement 1:
a. It is not possible that (G and not-J).
b. Statement 1 is true. It claims that it is not possible for event G and event not-J to obtain.
c. It’s equivalent to saying, e.g.: It is not possible for both John to be a bachelor and for John to be married.
7. Consider Statement 2, which commits the modal fallacy:
a. If G, then it is not possible that not-J.
b. This reads: If God knows that John will eat an orange tonight, then it is not possible that John will not eat an orange tonight.
c. But that ascribes logical necessity to a contingent event. It thus commits the modal fallacy.
d. (Using our “bachelor” analogy, the following is false: If John is a bachelor, then it is not possible for John not to be a bachelor. Wrong. Because it is possible for John to not be a bachelor. There is no logical necessity involved in John’s being a bachelor.
To see this argument, complete with modal symbols, go to the Internet Encyclopedia of Philosophy entry on “Foreknowledge and Free Will,” Section 6.
See also Craig and Moreland, Philosophical Foundations for a Christian Worldview, 517 ff..
Here G = God knows John will eat an orange tonight.
O = John will eat an orange tonight.
~ = 'not.'
◊ = 'possible'
⊃ = 'If..., then'
• = 'and'
The following statement is True: ~◊(G • ~O) [It is not possible that God knows John will eat an orange tonight and John not eat an orange tonight.]
The following statement is False (this is a modal fallacy): G⊃ ~◊~O [If God knows John will eat an orange tonight than it is not possible that John not eat an orange tonight. Or: If God knows John will eat an orange tonight than it is logically necessary that John eat an orange tonight. There is nothing logically necessary about John eats an orange tonight.]
Here's some explanation.
THE COMPATIBILITY OF DIVINE FOREKNOWLEDGE AND HUMAN FREEDOM
1. Some say, “If God knows what choices people are going to make, then we do not have free will in making those choices.”
2. Plantinga (and others) show that this reasoning is not logical. It commits a fallacy in modal logic.
3. Modal logic concerns the different “modes” of the verb “to be.”
a. The 3 modes of “to be” are:
i. Possibility
ii. Probability
iii. Necessity
b. Consider the statement: The coffee in this cup is hot. On modal logic, is this statement true?
i. Possibly? Yes.
ii. Probably? Yes (more or less).
iii. Necessarily? No.
c. If the statement The coffee in this cup is hot were necessarily true, then the coffee in the cup could not not be hot. But that is impossible.
d. No contingent statement can be necessarily true. To claim that is commit a modal fallacy.
4. Consider the statement John will eat an orange tonight. (Call this J)
a. Is this possible? Yes.
b. Is this probable? Yes (more or less).
c. Is this necessarily true? No. Because if it were, then John could not not eat an orange tonight.
d. What if the statement is false. Is it then necessarily false? No.
5. Let G mean: God knows John will eat an orange tonight.
6. Call this Statement 1:
a. It is not possible that (G and not-J).
b. Statement 1 is true. It claims that it is not possible for event G and event not-J to obtain.
c. It’s equivalent to saying, e.g.: It is not possible for both John to be a bachelor and for John to be married.
7. Consider Statement 2, which commits the modal fallacy:
a. If G, then it is not possible that not-J.
b. This reads: If God knows that John will eat an orange tonight, then it is not possible that John will not eat an orange tonight.
c. But that ascribes logical necessity to a contingent event. It thus commits the modal fallacy.
d. (Using our “bachelor” analogy, the following is false: If John is a bachelor, then it is not possible for John not to be a bachelor. Wrong. Because it is possible for John to not be a bachelor. There is no logical necessity involved in John’s being a bachelor.
To see this argument, complete with modal symbols, go to the Internet Encyclopedia of Philosophy entry on “Foreknowledge and Free Will,” Section 6.
See also Craig and Moreland, Philosophical Foundations for a Christian Worldview, 517 ff..
