Mackie's argument against the Free Will Defense is twofold. The first is essentially based on his rejection of incompatiblism, i.e., the notion that genuine free will is incompatible with determinism. He argues:
[I]f God has made men such that in their free choices they sometimes prefer what is good and sometimes what is evil, why could he not have made men such that they always freely choose the good ? If there is no logical impossibility in a man's freely choosing the good on one, or on several, occasions, there cannot be a logical impossibility in his freely choosing the good on every occasion. God was not, then, faced with a choice between making innocent automata and making beings who, in acting freely, would sometimes go wrong: there was open to him the obviously better possibility of making beings who would act freely but always go right. Clearly, his failure to avail himself of this possibility is inconsistent with his being both omnipotent and wholly good.
If it is replied that this objection is absurd, that the making of some wrong choices is logically necessary for freedom, it would seem that 'freedom' must here mean complete randomness or indeterminacy, including randomness with regard to the alternatives good and evil, in other words that men's choices and consequent actions can be "free" only if they are not determined by their characters. Only on this assumption can God escape the responsibility for men's actions; for if he made them as they are, but did not determine their wrong choices, this can only be because the wrong choices are not determined by men as they are. But then if freedom is randomness, how can it be a characteristic of will? And, still more, how can it be the most important good'? What value or merit would there be in free choices if these were random actions which were not determined by the nature of the agent? ("Evil and Omnipotence," 209).
Mackie's second response to the Free Will Defense is what he calls "the Paradox of Omnipotence." He writes:
But besides this there is a fundamental difficulty in the notion of an omnipotent God creating men with free will, for if men's wills are really free this must mean that even God cannot control them, that is, that God is no longer omnipotent. It may be objected that God's gift of freedom to men does not mean that he cannot control their wills, but that he always refrains from controlling their wills. But why, we may ask, should God refrain from controlling evil wills? Whv should he not leave men free to will rightly, but intervene when he sees them beginning to will wrongly? If God could do this, but does not, and if he is wholly good, the only explanation could be that even a wrong free act of will is not really evil, that its freedom is a value which outweighs its wrongness, so that there would be a loss of value if God took away the wrongness and the freedom together. But this is utterly opposed to what theists say about sin in other contexts. The present solution of the problem of evil, then, can be maintained only in the form that God has made men so free that he cannot control their wills.
This leads us to what I call the Paradox of Omnipotence: can an omnipotent being make things which he cannit subsequently control? Or, what is practically equivalent to this,can an omnipotent being make rules which then bind himself? ...
It is clear that this is a paradox: the questions cannot be answered satisfactorily either in the affirmative or in the negative. If we answer "Yes ", it follows that if God actually makes things which he cannot control, or makes rules which bind himself, he is not omnipotent once he has made them: there are then things which ce cannot do. But if we answer "No", we are immediately asserting that there are things which he cannot do, that is to say that he is already not omnipotent (Ibid., 209-10).