Free Will Remains a Mystery for van Inwagen

Van Inwagen dramatized his understanding of the indeterministic brain events needed foragent causation by imagining God "replaying" a situation to create exactly the same circumstances and then arguing that decisions would reflect the indeterministic probabilities. Here he mistakenly assumes that possibilities translate directly into probabilities.

He also mistakenly assumes that random possibilities directly cause human actions.
Now let us suppose that God a thousand times caused the universe to revert to exactly the state it was in at t1 (and let us suppose that we are somehow suitably placed, metaphysically speaking, to observe the whole sequence of "replays"). What would have happened? What should we expect to observe? Well, again, we can't say what would have happened, but we can say what would probably have happened: sometimes Alice would have lied and sometimes she would have told the truth. As the number of "replays" increases, we observers shall — almost certainly — observe the ratio of the outcome "truth" to the outcome "lie" settling down to, converging on, some value.

We may, for example, observe that, after a fairly large number of replays, Alice lies in thirty percent of the replays and tells the truth in seventy percent of them—and that the figures 'thirty percent' and 'seventy percent' become more and more accurate as the number of replays increases. But let us imagine the simplest case: we observe that Alice tells the truth in about half the replays and lies in about half the replays. If, after one hundred replays, Alice has told the truth fifty-three times and has lied forty-eight times, we'd begin strongly to suspect that the figures after a thousand replays would look something like this: Alice has told the truth four hundred and ninety-three times and has lied five hundred and eight times. Let us suppose that these are indeed the figures after a thousand [1001] replays. Is it not true that as we watch the number of replays increase we shall become convinced that what will happen in the next replay is a matter of chance.  ("Free Will Remains a Mystery," in Philosophical Perspectives, vol. 14, 2000, p.14)

Van Inwagen reveals that he clearly thinks that indeterminism directly results in actions. No wonder on his account that "free will remains a mystery!"

He repeated the argument more recently:

If God caused Marie's decision to be replayed a very large number of times, sometimes (in thirty percent of the replays, let us say) Marie would have agent-caused the crucial brain event and sometimes (in seventy percent of the replays, let us say) she would not have… I conclude that even if an episode of agent causation is among the causal antecedents of every voluntary human action, these episodes do nothing to undermine the prima facie impossibility of an undetermined free act. ("Van Inwagen on Free Will," in Freedom and Determinism, 2004, ed. Joseph Keim Campbell, et al., p.227)