Friday, December 22, 2006

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


At 1/01/2007 9:09 AM, Blogger Tom said...

Hi Alan-

Hope your holidays were great.

Have you seen Schwart's book online? You can find it downloadable here for free:

People have directed me to ch. 10 especially (which I have not yet read).


At 1/02/2007 3:18 PM, Blogger Alan Rhoda said...

Hi Tom.

Yes, the holiday were quite relaxing for us. Thanks for asking.

Thanks also for the link to Swartz's book. I'll take a look at it in the near future.

Blessing to you and Anita,


At 7/08/2007 9:08 PM, Blogger Jeremy Pierce said...


I know this post is old, but I'm just getting around to looking at it.

I'm not sure why I should accept 5. Assume exhaustive foreknowledge and truth about future contingents. I still don't think it's unpreventable that God knows that S will obtain at T. In possible worlds where S doesn't obtain at T, God doesn't know it. In worlds where it does obtain, God does. If those worlds are possible, then there are worlds in which my doing something to prevent S's obtaining at T seem to prevent God from knowing that S obtains at T. So I don't see why 5 should be true. It seems clearly false. If it's possible for S not to obtain, then it's possible for God not to know it.

At 9/21/2007 7:14 PM, Blogger leon said...

but only that it must be causally necessary (or unpreventable) if foreknown by God.

Yes this is valid if one accepts that humans are subject to moral or physical necessity; but we are not. We are free moral agents, we are sovereign and can be the first cause of our effects and are not subject to the strongest motive or causal necessity; therefore her argument is moot, Swartz's position still is cogent.

Strange argument coming from an open theist who denies causal or moral necessity.


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