In the comment box, "Dan" (profile not available) provided a proposed response from a non-Calvinist advocate of "Libertarian" free will (I assume that this is a hypothetical objection, not Dan's own objection):
God's infallibly knowing (with absolute certainty) that X will happen does not imply that X could not fail to happen (could not be otherwise), strictly speaking.The selection of the subjective mood "could" as opposed to the indicative mood "can" here is interesting. I suppose it is intentional.
The result, though, is a strange hybrid of moods. It is not "X would happen ... X could not fail to happen" or "X will happen ... X cannot fail to happen" but "X will happen ... X could not fail to happen."
Usually "would" or "could" are subjunctive mood forms that allow us to speak about hypothetical situations: "If someone hit me on the shins, that would hurt," as opposed to actual future situations for which we use "will": "When he hits me on the shins, that will hurt."
But what does this strange hybrid mood sentence mean: "God's infallibly knowing (with absolute certainty) that X will happen does not imply that X could not fail to happen (could not be otherwise), strictly speaking"?
How does changing the mood make a difference? It does not seem to have any obvious effect on the logic of the argument. In other words, just as the fact that it is absolutely certain that X will happen implies that X cannot be otherwise, so also the absolute certainty of X happening also implies that X could not be otherwise.
There's possibility that the "could" could be in reference to a situation other than the actual world. In which case, the point seems moot. In this actual, real world, it cannot be otherwise, whether or not it could be otherwise in some other imaginary world.
The proposed objection continues:
[T]here is nothing incoherent about holding these four claims:Notice the mood change again from (A) and (B) which have the indicative mood to (D), which has subjunctive mood.
A) John will choose X.
B) God knows that John will choose X.
C) Although John will choose X, he has the power to refrain from choosing X (securing a key libertarian condition for freedom).
D) If John were going to refrain from choosing X, then God would have always known that John will refrain from choosing X (instead of knowing that John will choose X, as he in fact does).
(C) probably holds the key to understanding and rebutting this objection. In (C), the mood is indicative throughout. The claim is that John has (not would have) the power to refrain from doing X. But given (B), it is not possible for John to refrain from doing X. He does not have that power. Thus, (C) cannot be held together with (B).
(D) is a hypothetical situation that doesn't really affect (C). If John were going to refrain from doing X, then God would have known that John was going to refrain from doing X. That may be true, but it is not our situation.
I suspect that (D) is raised, because we attempt to prove that (C) is wrong by pointing out that if John refrained from doing X, then God's knowledge would be wrong. The response is that God's knowledge would not be wrong, it would be different.
But our point is really more limited. Our point is not whether in a world where God had foreseen that John would refrain from doing X, John could refrain from doing X (we agree that John could refrain in such a world), but whether in this world where God has foreseen that John will do X, John can refrain from doing X. We could full agree with (A), (B), and (D), it's just (C) that is problematic.
In other words (D) does not allow (B) and (C) to be brought into harmony with one another. Instead, (D) suggests that (C) can be true so long as (B) is false. But that's close to being the very definition of a contradiction. Thus, it is incoherent to hold those four propositions, since holding to (C) contradicts (B), as can be seen from (D).