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(For my MCCC Philosophy of Religion students)
Philosopher-atheist J.L. Mackie constructs an argument from evil intended to show the incoherence of theism. One cannot, thinks Mackie, simultaneously affirm the following three propositions (known as "Mackie's Triad"):
1) God is all-powerful.
2) God is all-good.
3) Evil exists.
Just as one cannot simultaneously affirm:
1) John is a bachelor.
2) John's wife's name is Linda.
1) Object X is square.
2) Object X is circular.
With this last example, we see that there is no possible world where an object, X, can be at the same time square and circular. There is, e.g., a possible world where a talking sponge can exist; i.e., it is logically possible that a talking sponge can exist. The term "talking sponge" is not logically impossible (logically incoherent). But "square circle" is. Mackie's claim is that theism, the idea of an all-powerful, all-good being, is incoherent on the existence of evil. That is, one cannot imagine a possible world where an all-powerful, all-good being coexists with evil.
As convincing as Mackie's argument sounds, it is false. To defeat Mackie all one would have to show is that there is a possible world (or there are possible worlds) where the existence of an all-loving, all-powerful being is compatible (coherent) with the existence of evil. This has been done. (Note: this is modal logic stuff.)
Nearly all philosophers, to include atheists such as William Rowe, believe Alvin Plantinga has defeated Mackie's logical argument, and that therefore theism is not incoherent. All Plantinga needs to do is show that there is a possible world in which Mackie's Triad can be affirmed. If Mackie's Triad was logically inconsistent then there could be no possible world where an all-powerful, all-loving being existed with the existence of evil.
Plantinga does this by showing:
1) It is possible that God has given persons libertarian free will.
2) It is possible that God has counterfactual knowledge.
3) It is possible that transworld depravity exists.
"Libertarian free will" is: making a choice (such as, e.g., a moral choice) that is not fully reducible to antecedent causal conditions.
To say that God could have counterfactual knowledge is to say that God knows the truth value of future conditional statements that describe possible states of affairs. (Note: if one thinks that God's counterfactual knowledge eliminates free will they have just made an error in modal logic - see here.)
By "transworld depravity" Plantinga means: in all possible worlds human agents will commit at least one evil act.
If, then, there is a possible world where libertarian free will exists and God knows what choices John will make, and knows that John will choose, on at least one occasion, evil, then God cannot make a world where John is faced with that choice and chooses good. This is because it would violate John's free will.
But what if, Mackie asks, God made a world where all persons on all occasions chose good? Plantinga responds by saying that it is possible transworld depravity exists. If so, then we have a world where one can affirm the existence of an all-powerful and all-loving (as well as all-knowing) God exists, as does evil.