Tuesday, April 16, 2013

Pruss, PSR, and QM

In the discussion of the Leibnizian cosmological argument for God, Tyler mentioned a book by Alexander Pruss (of Prosblogion fame), The Principle of Sufficient Reason, A Reassessment. In particular, Tyler said he thought Pruss had answered all the possible quantum mechanical objections to the Principle of Sufficient Reason (PSR). I have finally gotten a chance to look at Pruss's chapter on QM. It seems to me that he succeeds - much too well.

Pruss has a good grasp of the quantum mechanical objection. He even presents a version of the famous EPR-Bell inequality paradox. He goes on to suggest several ways of reconciling QM with the PSR, including non-local causation, or backward-in-time causation, and hidden variable theories. With regard to the latter, he acknowledges that Bohmian theories run into problems with relativity, and are not a really satisfactory replacement for standard quantum field theory. But, he says, it is possible to show that some such theory could, in principle, explain the QM results.  

For instance, take a neo-Leibnizian theory that says that every point of space is a monad, and this monad has encoded within it a list of all the events that will happen throughout time at that point and through an internal causal process it goes deterministically through these events as time passes. 
Now, it seems obvious to me that this solution achieves too much.  For this could be said of any conceivable pattern of events in any conceivable universe. No matter how random, lawless, and chaotic, those events could be described in Pruss's monad theory as deterministic and causal. So this solution makes the PSR trivial, and therefore uninteresting. If every possible pattern of events satisfies the PSR, then the PSR has no content.

If the PSR means anything at all, then it needs a more rigorous notion of "reason" and "causality" than Pruss is employing here.


  1. And this seems to me the problem with all such grand metaphysical claims. Just arbitrarily add another assumption, and your metaphysical principle can be made to fit any reality whatsoever. Which as you point out makes the principle useless.

  2. Well, I don't think that's fair to Pruss. The PSR is a metaphysical principle, not a scientific theory. If it can fit with every possible pattern of events - well, that's what deep metaphysical principles are all about. It would be a point against the PSR if you could conceive of a universe where the PSR does not hold.

    For example, consider the Law of the Excluded Middle - stating that it is either True or False that x exists (for any x); there is no third option. Now this principle cannot be proven (any proof will use it, so will be circular), and no possible state of affairs in any possible world will fail to fit it (because we construct "possible worlds" precisely by using the LEM). Nevertheless, it is a widely accepted metaphysical principle, one I myself accept.

    Now I think the PSR is bullshit. Pruss has a valid point, however, in that the observations that lead us to accept an acausal interpretation of QM are consistent also with a causal/deterministic interpretation. Pruss totally misses the point that the fact that it what matters is that we can concieve of a world that does not obey the PSR, namely one following an acausal interpretation of QM (a Copenhagen or Contextual interpretation). This precisely means that the PSR is not a deep metaphysical principle, applying in all possible worlds, like the LEM does. So his valid point totally misses the mark, not comprehending the nature of the QM objection - it isn't that the Copenhagen/Contextual interpretation is true (I personally think it isn't), but rather that it's possible. Once the PSR is removed from being an overarching metaphysical principle, you need to argue positively for it applying in our world. Which, given Pruss's own argument, is rather difficult, since like the LEM it fits any pattern of events and thus you can't argue empirically for it; yet we've just established we can't argue philosophically for it, since it isn't necessarily true...

    Incidentally, a lengthy reply to Pruss is here:

    I find the author to be very worth reading, from earlier posts, ... but haven't read this piece.

    Last, but not least - Pruss's key paper is really based on the implicit denial of the Humean/regulatory understanding of "contingency". I'm gonna write about that in length, at some point... if I can find the time to.... I haven't read his book, but I suspect it does the same.



    1. A more precise link:



  3. I think the charitable way of reading Pruss (which is also, incidentally, the correct way as far as I can tell) is to say that Pruss is arguing that 1) the PSR is not an empirical claim, and 2) it is not the case that there exists any empirical evidence which acts as a defeater for the belief in the PSR. "If every possible pattern of events satisfies the PSR, then the PSR has no content." - that objection would only work, it seems to me, if one were claiming that the PSR were an empirical principle (rather than either a metaphysical principle, or a modal-logical principle). This would perhaps have been clearer if you had read the rest of the book along with that chapter, but one can only do so much I suppose. Still, I congratulate your cracking open the book. I look forward to reading more of your thoughts on the subject.

  4. OK, guys, I get the point about PSR not being an empirical principle. But Pruss's formulation of it still leaves it useless for philosophical argument.

    Consider the Leibnizian Cosmological argument, for example.
    (1) Everything that exists contingently has a reason for its existence.
    (2) The universe exists contingently.
    At this point, following Pruss, I can argue that the initial singularity is a monad, and this monad has encoded within it all the events that will happen, and the first thing on the list is "Bang really big." (Or, "Let there be light"?) So now I have a "reason" for the Big Bang, and the cosmological argument is at a stand-still.

    I suppose Pruss must at some point discuss what he means by a "reason." But if it includes his monad-with-a-list type reason, it's clearly too weak to be useful. What he's saying is that if I want to know the reason that X happened at location Y, it's enough of a reason to say "Because X was the next thing that was going to happen at Y." This is the "shit happens" theory of explanation. You might was well say the reason is "Because God wanted it that way" and have done.

  5. I agree with your second point, but not your first one.

    As I understand the discussion, Pruss didn't raise the deterministic-monads interpretation of QM to defend the PSR but rather to counter the claim that QM implies indeterminism which in turns amounts to some things happening for no reason. Pruss rightly notes that a deterministic account is possible so the QM objection, as he understands it, is on shaky ground. As I said, his reply is problematic. But his reply does not amount to defending the PSR itself. His monadic interpretation does not suffice to provide a PSR - since he'd still need to provide a reason for why these monads exist. I believe he would argue that ultimately, the PSR must apply to his monadic interpretation for it to be coherent, and the PSR implies that just putting a Big-bang on the list is certainly not enough; you'd need to add God for the PSR to be satisfied.

    Now I do agree with your second point - that the meaning of "reason" is a pivotal question. My impression of Pruss is that "reason" for him is deeply-tied to rational necessity. It is as if rationality is above reality, dictating it; e.g., the fact that B is listed to follow A means that it will "magically" do so, as if the is some essence of something that gives it power to shape reality. In such a view, if the PSR fails then it is as if nothing makes sense anymore, as if the laws of logic itself have been violated. So the PSR is almost a logical necessity.

    For the Humean - like myself - "reason" is deeply tied to fitting in with a general pattern. The "reason" current flows down the wire is that electrons are attracted to positive charges. On this view stochastic patterns will mean the PSR fails, but even without them the PSR will fail when we reach the ultimate, most general, pattern. So the PSR is almost an impossibility, an attempt to invoke a pattern more general than the most general pattern.

    That's how I, currently, see the mental process behind the verbal disputes. I may be mistaken, of course, especially about Pruss's views.



  6. Suppose that the monad theory were the only way to square QM with the PSR (it's not). Then PSR+QM would imply the monad theory. But the monad theory is far from being a trivial claim--it is a highly substantive (and, I think, false) claim about the structure of reality, viz., that there really do exist these monads whose states deterministically explain everything to come. So PSR+QM would imply a highly substantive claim about the structure of reality, which we surely can't get from QM alone. How can the PSR be trivial in that case?

    As for the proposed monadic explanation of the universe's existence, that just points to the famous gap problem with the Cosmological Argument. The Cosmological Argument only gets you to a necessary being that is a first cause. (The monad in your story had better be necessary, or we can ask why it exists.) Further argument is needed (and has been offered by various authors) that the necessary being is something like God.

    In other words, the PSR gets you the bare idea that every contingent fact has an explanation. Further argument -- and, in many cases, empirical observation -- is needed to try to identify what that explanation is.