In November 2009 the Center for Ethics and Values in the Sciences at the University of Alabama Birmingham hosted a conference entitled Does Scientific Naturalism Exclude Metaphysics? The speakers were Michael Friedman, Andrew Melnyck, Ron Giere, Mark Wilson, Don Ross, Daniel Dennett, J T Ismael, James Ladyman, and Paul Humphries. The conference was video taped and the videos are now up on YouTube here courtesy of Sarah Vollmer and her graduate student Morgan Anders who are also in the process of making a short documentary film on the issues raised.
The conference focused on Ladyman and Ross’ new book Everything Must Go: Metaphysics Naturalized where they argue, first that scientism is true and second that a lot of contemporary metaphysics, even from philosophers who claim to be naturalists, physicalists, and scientismists (Armstrong is cited as an example), relies on a fundamentally misguided and outdated conception of scientific reality as consisting of little billiard balls flying around in space banging into each other, you know basically the idea that Democritus had 2,500 years ago. Scienticism is the view that science, in particular physics and the methods it employs, is the only real way to know about the world. A priori reasoning on this view is no good, especially when it is detached from science or especially when it employs this outdated model of the scientific model. They argue that the proper role of metaphysics is that of elucidating the connections between the various special sciences so that they form a unified picture of the nature of reality. This is a task that falls to no specific science and so can be called metaphysics (they cite as an example the claim that chemistry unified is physics and physics unified is metaphysics).
My own reaction is to be sympathetic to the criticism of philosophers who try to derive conclusions about the actual world from current a priori reasoning. Given the track record it is far from clear that a priori intuitions are a good guide to the nature of the actual world. They are however a fine guide to the possible worlds. A priori reasoning fills out the space of possible and impossible worlds and science then locates the actual world in that space. The “fictional world” that occupied David Lewis, David Armstrong, as well as philosophers like Locke, Hume, and Kant, is a perfectly respectable possible world and is interesting in so far as it is a live option, which roughly means that it hasn’t been ruled out by scientific inquiry. The main thrust of Ladyman and Ross can then be seen as an argument that science doesn’t bear this picture out and so naturalistically minded philosophers should stop thinking about one set of possible worlds. But nothing in the argument suggests that a priori reasoning about a different set of possible worlds. In fact we need the a priori reasoning about possibilities to make sense of the empirical data and this is the way we will ultimately identify the the mind with the brain , for instance. .
What we get from this kind of picture is a two-dimensional view on which a priori reasoning gives us the primary intensions of statements and science gives us the secondary intensions, or to put it in more Kripkean terms, the job of science is to reduce the epistemically possible to the metaphysically possible. This is still an empiricist position broadly construed since the claim is that for beings like us the only way to know about the actual world is via empirical means. In fact i would count this as a scientismist position. This is perfectly consistent with the claim that an ideal agent who knew all of the facts would be in a position to know about the actual world in an a priori manner.