Yale’s Michael Della Rocca has a paper out in support of the principle of sufficient reason (PSR). Della Rocca, whose expertise is on the Rationalists, begins the paper with a humorous shtick about how he is writing the paper against his better instincts, since he knows how little credibility the PSR has in contemporary mainstream philosophy. [UPDATE 4 August 2010: I neglected to tip hat to Sympoze.]
His strategy in the paper is to push the burden of proof back onto those who dismiss the PSR.
To begin, he points out that we all make common use of what he calls an “explicability argument”. This is an argument where “a certain state of affairs is said not to obtain simply because its obtaining would be inexplicable – a so-called brute fact.” One of Leibniz’ examples was that of a scale or balance: if we hang equal weights on each side, we infer that the balance will remain still. That is because there is no reason why one side should hang down; if one side were to hang down, it would be inexplicable.
Della Rocca goes on to give many examples of explicability arguments with an emphasis on their role in contemporary philosophical debates: e.g., arguments that, say, consciousness or modality is dependent on other features (if not, they would be inexplicable).
Now take the case of existence: what explains why the things that exist exist? Unlike the other examples, taking on this question makes a “global” commitment to the PSR.
Della Rocca notes you can certainly say explicability arguments are useful in some cases, but not in the case of existence. Presumably this is what the opponent of the PSR would say. But then, he says, he has a right to ask for a principled argument regarding where you draw the line between the appropriate usage and the full-blown PSR.
He doesn’t want to hear appeals to intuition. The embrace of explicability arguments elsewhere is enough to get one to take seriously the possibility that it should apply to the case of existence as well. So we want a positive reason for its inapplicability here.
Another strategy might be to point to bad consequences of embracing the PSR, such as the Van Inwagen/Bennett argument that it leads to necessitarianism. Della Rocca says even if this is correct it doesn’t suffice as a principled case against the applicability of explicability arguments in the case of existence (in other words, if the PSR does mean necessitarianism, then so be it).
Pointing to the apparently anti-PSR implications of quantum mechanics is similarly missing the point about needing an argument for where the line is drawn on using explicability arguments.
So, until a line is drawn in a principled and non-arbitrary way, Della Rocca argues the PSR is alive and well.
My take: Della Rocca does not discuss further how the implications of QM might be involved in this debate, but my first though is that this is where I would develop an argument about drawing the line. I might argue that existence and certain other problems closely tied to it (conscious experience) are unavoidably concerned with quantum mechanics. On the other hand, many other problems in the macroscopic world do not depend on the specifics of QM, and thus can legitimately make use the explicability argument.
I would note that even in the world of QM, a weaker cousin of the PSR can still be potentially used: the things in our world depend on (but are not completely explained by) something else.