Here’s a very nice comment in response to Steve French. I wanted to share it, well, because I agree with almost everything Wallace says.
“I’m going to test our hosts’ patience by (pretty much) reposting the last thing I wrote on the earlier discussion (by the time I got around to posting it, that discussion had got rather buried under the march of NewAPPS discussion!)
There’s no logical reason why there shouldn’t be some question in metaphysics that can’t be answered satisfactorily without high-level professional training in both physics and philosophy. If so, it doesn’t become any less the case because the economic framework of 21st-century academia makes it difficult or impossible for anyone to obtain that level of training. The universe is not obliged to conform its complexities to our funding models.
Having said that (and here I’m really being speculative) I actually think a lot of the physics-based problems I run into in conversations with metaphysicians aren’t so much because people are trying to do physics-neutral metaphysics; they’re because people are trying to do metaphysics which relies on a particular conception of physics that isn’t right. That conception is something roughly like this (here I’m grateful to Eleanor Knox for this way of putting things):
The world is a swarm of particles, following trajectories in a spacetime that is locally Minkowskian but on larger scales is curved. The particles are pointlike objects, and come in relatively few varieties: red quarks, anti-neutrinos, and Higgs bosons, to name a few. They interact by colliding with one another, or by forces acting between them, or maybe by influencing and being influenced by some fields (which are either objects whose parts occupy spacetime regions, or are fancy ways of talking about certain properties of spacetime points). Macroscopic objects are swarms of these particles.
That picture of the world isn’t far off what you might get from a popular book on particle physics (though it’s much more sophisticated vis-a-vis spacetime). And according to mainstream contemporary physics, it is systematically and utterly wrong. Furthermore, it was never really thought to be right: it’s not even really a good account of what people thought in the 19th century.
So my feeling (simplifying grossly; hey, it’s a comment in a blog post) is that there are two kinds of metaphysics that can reasonably be done. There are projects which really do engage with the details of the physics, and those are continuous with philosophy of physics. And there are projects which are fairly neutral on the details of the physics: having a general idea of the way physics tends to work is a very good idea, but you don’t need quantum field theory. (Personal identity is my paradigm example of a problem like this; maybe natural laws, too.) The projects that run into trouble are the ones that actually are dependent on details of the physics, and rely on a badly wrong conception of physics.
Now to be fair, identifying when a project is so reliant is tough. One rough-and-ready criterion: if your approach is dependent on the correct interpretation of quantum theory, I doubt you can pursue it without getting into the details of the physics. Another: if you actually need to say things about subatomic particles, you probably can’t avoid learning particle physics. Jason gives the example of using quark colour in a paper: if you genuinely need to talk about quarks, I don’t think you can avoid learning about what quarks actually are; if you can get away with talking about tables, do that instead. Trouble only ensues if you end up talking about a popular-science conception of quarks. A third (perhaps) is the one I suggested above: ask if your theory still works if all matter is made up of earth, air, fire and water, mixed to varying degrees. If it doesn’t, but it does on the picture of the world I give above, then worry.
It’s worth observing that the special sciences don’t have any problem being largely neutral about fundamental physics. I don’t see much reason why we can’t have comparably physics-neutral programmes in metaphysics. But if the project you want to pursue can’t be done in that emergent, physics-neutral way, because it’s concerned with the true and ultimate structure of reality, then I don’t see any way to pursue it sensibly without engaging with modern physics in full.”