This is my take on a couple terms that get bandied about in the science/religion discussions:
Philosophical Naturalism: the belief that the natural world is all that there is.
Methodological Naturalism: the belief that the natural world is all that is accessible for scientific inquiry.
To me, that is clear enough, though I freely admit that some may object that these definitions are too simplistic.
Any theist obviously rejects philosophical naturalism. To us, the natural world is not all there is. Indeed, it is not even the bulk of all there is—it is but a shadow of better realm, a supernatural realm. Being a theist, or even a deist, means you accept at least God himself, even if he is now dead or has moved to the suburbs, as a supernatural entity. It is possible to be a theistic evolutionist, but it is not possible to be a theistic philosophical naturalist.
The problem has always been, of course, that methodological naturalism is confused with philosophical naturalism. But methodological naturalism is a far, far weaker statement. Methodological naturalism tells us that science is limited, in scope, to the study of the natural world. Theists can and do support methodological naturalism.
In general, when science is successful it supports but doesn't prove philosophical naturalism. When science fails, especially for extended periods, it doesn't disprove philosophical naturalism—but it shakes its foundations a little.
For example, take origin of life research. Should it prove successful, it will be a big feather in the cap of the philosophical naturalists. Should it, after decades perhaps, produce no substantive progress, then that would be a kick in the shins of philosophical naturalism.
Or my beloved cosmological fine tuning. Should another universe be detected, or should the fine tuning be demonstrated to be an illusion, it would be in the win column of philosophical naturalism.
So to reiterate:
- When science is successful it tends to support philosophical naturalism—though theists will still believe that the success of the science just as readily demonstrates the beauty of creation.
- When science fails it tends to weaken the philosophical naturalism position, though proponents will always argue "give us more time."
- But the juicy question is: can a scientific success ever undermine philosophical naturalism?
Well many people think so. They will tell you that ID is a scientific endeavor that undermines philosophical naturalism. Here we face a subtlety. Because while science and methodological naturalism are often used (as I have) interchangeably, a better way to look at it is:
Science: the only accepted, time-tested set of procedures (the scientific method) used by the methodological naturalist (who may or may not be a philosophical naturalist.)
Therein lies the problem for both the IDist and the multiverse proponent: while they are discussing aspects of the natural world (the bacterial flagellum, parallel universes) that in principle fall under the purview of methodological naturalism, nobody has devised a way to use the scientific method to test their claims. IDists and multiverse-ists are, in some sense, methodological naturalists without portfolio. ID cannot undermine (or bolster) philosophical naturalism, at least not scientifically, because it cannot get into the game.
OK, but can science, if successful, ever truly undermine philosophical naturalism?
I think one scenario that I have often mentioned is as close as we can ever hope to come. And that scenario is the development of a fundamental theory that explains the physical constants. It is ironically, the exact opposite of the way ID is usually argued: it is not the demonstration of how unlikely our habitable universe is that would be a major success, but the demonstration that it was inevitable. That some future "theory of everything" had habitability built into it would, in my opinion, be the most difficult successful scientific result for philosophical naturalism to swallow.
Let me end by referring once again to Uncommon Descent, because it is their "about" statement that got me thinking along these lines. They write:
Uncommon Descent holds that…
Materialistic ideology has subverted the study of biological and cosmological origins so that the actual content of these sciences has become corrupted. The problem, therefore, is not merely that science is being used illegitimately to promote a materialistic worldview, but that this worldview is actively undermining scientific inquiry, leading to incorrect and unsupported conclusions about biological and cosmological origins. At the same time, intelligent design (ID) offers a promising scientific alternative to materialistic theories of biological and cosmological evolution -- an alternative that is finding increasing theoretical and empirical support. Hence, ID needs to be vigorously developed as a scientific, intellectual, and cultural project
I realized after reading this pedantry for maybe the hundredth time, I have no idea what they are talking about. What is materialistic ideology? Is it philosophical naturalism or methodological naturalism? Or is it something else altogether? I could argue that their statement in toto is not consistent with either view, so that it must be that "materialistic ideology" is neither philosophical nor methodological naturalism. It must be something else. I don't what—but maybe someone who speaks UDish will be so kind as to offer an explanation.