Foreknowledge, Free Will, and "The Modal Fallacy"

In his Internet Encyclopedia of Philosophy article entitled "Foreknowledge and Free Will," Norman Swartz defends the view that divine foreknowledge is compatible with human free will and contends that arguments for incompatibilism inevitably commit a certain modal fallacy. I think he's wrong on both counts, but I want to focus here just on the second claim.

First, I need to clarify that 'free will' is to be understood in the libertarian (indeterminist) sense and that 'divine foreknowledge' is to be understood as the view that God from the beginning infallibly knew of every possible state of affairs S and of every moment of time T, either that S will obtain at T or that S will not obtain at T.

Now, what is this "modal fallacy" that Swartz makes so much of? Basically, it's the inference from "Necessarily, if p then q" to "If p then, necessarily, q." In other words,

Nec(If p then q)
∴ If then Nec(q)

This is indeed a modal fallacy. That p entails q and that p happens to be true, does not imply that q is necessarily true, i.e., true in all possible worlds. The relevance of this fallacy to the foreknowledge-free will debate lies in the thought that the incompatibilist argues as follows:

1. Necessarily, if God knows that S will obtain at T, then S will obtain at T.
2. God knows that S will obtain at T.
3. Therefore, necessarily, S will obtain at T.

And, indeed, if the incompatibilist were limited to arguing in this fashion, Swartz's charge that she must commit a modal fallacy would be right on target. But to suppose that the incompatibilist is limited to arguing in this fashion is quite uncharitable. For one thing, the incompatibilist doesn't need to show that S's obtaining at T is logically necessary, but only that it must be causally necessary (or unpreventable) if foreknown by God. The following argument, in other words, is - if sound - sufficient to establish the incompatibilist thesis, and it does not commit the modal fallacy identified by Swartz:

4. Necessarily, if God knows that S will obtain at T, then S will obtain at T.
5. Unpreventably, God knows that S will obtain at T.
6. Therefore, unpreventably, S will obtain at T.
   

Premise (4) follows from the thesis of divine foreknowledge (understood in the sense defined above) together with God's essential omniscience (i.e., the claim that God knows all and only truths in all possible worlds in which God exists). Premise (5) follows from the supposition of divine fore-knowledge (i.e., that God knew in the past that S was going to obtain at T) together with the plausible claim that the past cannot be altered. From these (6) seems to follow. I should mention, though, that many scholars have challenged premise (5) claiming that God's past noetic states are not unpreventable "hard facts" but rather "soft facts" that depend for their content on what happens in the future. I think this objection is misguided, but I'll set that aside. My point here is simply that incompatibilist arguments need not commit the modal fallacy described by Swartz.

One last point. We have seen, the incompatibilist can avoid the modal fallacy by invoking in the conclusion a weaker type of necessity than logical necessity. The important point is that there are different modes of necessity, and these are distinguished by their scope. Logical necessity has as its scope all logically possible worlds. Physical necessity has as its scope all logically possible worlds with the same physical laws as ours. Temporal necessity has as its scope all logically possible worlds having the same history up to some specified point. Note that types of necessity other than logical necessity add further qualifications or restrictions of scope. One can always validity infer from a broader scope to a narrow scope contained in the broader one. Thus, if something is logically necessary, then it is also physically necessary. But one cannot legitimately infer from a narrower scope to a broader one. This is the problem with the so-called modal fallacy. In the premise

Nec(If p then q)

the type of necessity that pertains to q is not the type of necessity indicated by the operator "Nec()" but a necessity of a narrower sort because the antecedent "if p" adds a further qualification. Thus, the necessity pertaining to q is what we might call p-necessity, which has as its scope all logically possible worlds in which p is true. From this we can validly infer

If p then p-Nec(q)

but not

If p then Nec(q)

for the latter involves moving from a narrower scope to a broader one.