I really appreciate all the comments on my Fine Tuning Argument for Naturalism (FTAN). Ben Yachov made the excellent point that a constant tweaking by God of the natural laws would itself appear to be a sort of natural law, and not a miracle. Tyler asks about the amount of parameter space that would be gained by God's intervention, and would it really be as large as I am suggesting. He writes, "...we need the numbers to argue concretely." Together, these two criticisms make a serious challenge to my argument. I have tried to answer them in brief in the comments, but here I will try to give a fuller account, complete with numbers.
Ben's point is a very cogent one, and requires careful consideration. If, for example, the strong force were too weak for atomic nuclei to hold together, and yet God held them together, how could that be seen by physicists as anything but a natural law - a strong force, or an additional force, strong enough to hold nuclei together? I feel that, in principle, an omnipotent God ought to be able to allow life to exist in a non-law-like way. But as I have nothing other than a law-like universe to refer to for examples, I think it would be hard to argue this point. So, I will (partially) concede Ben's point. Unfortunately for me, this means my FTAN in its generic form can't go through. Still, I think I can save the argument by appealing to particular cases where Ben's complaint can be circumvented.
First let me point out that, although science has revealed a unified and consistent picture of natural laws, that need not have been the case. A couple decades ago there seemed to be a developing contradiction about the age of the universe. Astrophysicists modeling the evolution of stars were coming up with ages for those stars that were longer than the age of the universe that cosmologists were suggesting. Of course, creationists got very excited about this and began crowing about how the Big Bang model was inconsistent and so forth. In the event, the star ages got revised downwards and the universe age upwards, so that today there is no longer any contradiction. But that need not have happened. It could have happened that astrophysicists, applying their methods, came up with ages that were much larger than the age that cosmologists, using their own distinct methods, allowed for the age of the universe. Thus science would have had a conundrum. Methodological naturalism would prevent scientists from resolving this conundrum by postulating supernatural intervention. But the conundrum would be available for anyone who wanted to argue for a supernatural being or cause.
What does this have to do with Ben's point? Well, in a universe that runs by rigid natural laws, there is nothing surprising in the fact that the laws give consistent answers to questions like "How old is the universe?" But in a universe created by an omnipotent being, such consistency is surprising - because there are so many other ways God could have created the universe. For instance, by injecting stars that are already in an advanced stage into an already-expanding universe. In this, the young earth creationists are logically correct: God could have created a universe that was only 6000 years old, but which had the "appearance of age." And there is no particular reason that the ages, as deduced from different sources using the (incorrect) naturalistic assumption, would agree with each other. So, in addition to Ben's suggestion that miraculous intervention appears as natural law, there is another possibility: that a miraculous intervention would show up as a conundrum, i.e., an apparent contradiction between natural laws. I will give some other examples below.
The second general point to make is that there is nothing in our known natural laws - apart from the supposed fine tuning of the parameters of those laws - that seems to prefer life to non-life. Given an omnipotent God, it is possible that there would be either special natural laws for life that differ from those for non-living things, or special exceptions to otherwise uniform natural laws that permit life in places where it would otherwise be impossible. The latter would (per Ben) appear as natural laws - but laws of a very particular type (see below) and of a type we do not see in our actual universe.
OK, now on to three specific examples to show what I mean, and to give some numerical values to answer Tyler.
In discussing these examples, I will use the term de-tuning to describe the amount of parameter space that is opened up by the God hypothesis. Let's call the naturalistically life-permitting range of the parameter the n-range, and the range of the same parameter for which an omnipotent God could allow life (like ours) to arise the g-range. Then define the de-tuning to be the ratio n-range divided by g-range. According to the assumptions of the Fine Tuning Argument, taken over into the FTAN, this ratio represents the probability that the parameter will fall in the naturalistically life-giving range, assuming theism to be true, and all other things being equal.
(Of course, God could presumably allow life unlike life on Earth to arise in a much wider range of parameters, so this ratio is a conservative estimate of the de-tuning.)
1. Cosmological Constant: The cosmological constant is a favorite target of fine-tuning arguments. It is a constant that appears in Einstein's General Relativity equations. A non-zero cosmological constant acts like a uniform energy density spread throughout space, and therefore is a candidate for the so-called "dark energy" of the universe.
If the cosmological constant is large and negative, the universe will re-collapse on itself too fast for galaxies, and therefore life, to form. If the cosmological constant is large and positive, the early universe will expand too fast for galaxies to form - the primordial hydrogen spread throughout space will not have time to collapse under its own gravity.
Now, it turns out that no matter how large the cosmological constant is, the expansion will not tear apart structures that have already formed. So, an omnipotent God could create a universe with a positive cosmological constant, then miraculously collect enough primordial matter into one place for a galaxy to form. Life could then evolve naturalistically without any further intervention. It doesn't matter how large the constant is, this scenario remains possible for any positive value. Thus, the g-range is, in principle, infinite. But if we follow Collins and use the Planck scale as the reference range, then we get a conservative estimate for the de-tuning that is essentially the same as his result for the fine-tuning, which he gives as one part in 10120.
This example shows that the de-tuning is not just a matter of slightly expanding the range of some parameter, as Tyler seems to suggest. The g-range is vastly larger than the n-range, and therefore (by the logic of the Fine Tuning Argument) the probability of finding our universe lying in the n-range is vastly smaller under the theistic assumption.
2. Origin of life on Earth: It is possible that natural laws allow life to exist, but don't allow life to arise from non-life. In this case, God could initiate life on Earth by "seeding" Earth with the original life-form(s), which then could go on to evolve into a variety of different forms in a naturalistic way. There is obviously an infinite number of ways this could happen: God could start with just a single reproducing cell, or she could seed the earth with a whole range of flora and fauna.
There is no way to capture the infinite possibilities in a single number, but we can get a sort of very conservative limit on the de-tuning by considering the age of the Earth. It seems that naturalistic processes required 3 billion years to produce complex life. The Sun is expected to shine for about 10 billion years before becoming a red giant. Naturalistically, then, we should expect the age of the Earth to be between 3 and 10 billion years at the time intelligent life arises. But under theism, it could be anything from nearly zero to 10 billion years. This gives a not-very-impressive detuning of 0.7.
3. Distance of the Earth from the Sun: The earth-sun distance doesn't make a good candidate for a fine-tuning argument: given the large number of planets in the galaxy it is highly likely that some of them will be at the "right" distance from their star for life to arise: the region called the "habitable zone." (Nonetheless, some folks still cite it.) However, we can turn it into a de-tuning parameter as follows.
A planet too close to its star for life could be miraculously protected from the excess radiation by a sort of blanket surrounding the planet. At this blanket, energy disappears from the universe. Elsewhere, I assume, the laws of physics are the same as in our universe, so energy is conserved everywhere except at this blanket. Thus, the planet could be much closer to the star than the habitable zone, and still contain life. Moreover, the blanket could instead provide energy to the planet, and so life could exist on a planet farther from the star than the habitable zone, too. Indeed, such a planet would not need a star at all, so it could be wandering through interstellar space.
To estimate the de-tuning in this case, note that the Wikipedia page cites the habitable zone as about 1.4 AU wide, providing the n-range. The g-range extends from the surface of the star out into interstellar space; we can take it be half the average distance between stars, or about 2 light-years. So the de-tuning is about 10-5. (Again, I am making a very conservative estimate. There is no reason such a planet couldn't wander about in inter-galactic space. Allowing that scenario would give a much more impressive de-tuning factor.)
4. Chemistry for life: Lastly, consider the possibility that the laws of chemistry are not (fine) tuned for life, but God changes the rules so that atoms and molecules behave differently when in living things than in non-living things. Per Ben's dictum, this would appear to us as a natural law: the chemistry of living things is fundamentally different from that of non-living things. (It would give rise to conundrums, too. Molecules are constantly passing from inside the body to outside via respiration, perspiration, etc., and we would have no naturalistic way of explaining the chemical change when they do.)
I don't know how to put a de-tuning value on this possibility, but obviously there are many more ways life could exist under this scenario than in a world with a single unified set of chemical laws.
I think I have shown that divine interventions vastly expand the possibilities for life to exist in the universe. These interventions could show up either as conundrums - apparent contradictions in the laws of nature, as in the first two cases above - or as natural laws that single out living things in a special way. Strictly speaking, only the former count as expanding the parameter ranges that allow for life to exist, since the latter may be considered as "naturalistic" scenarios. But special natural laws for life is just what we don't see in our universe: we don't see planets wrapped in energy-destroying bubbles, or see molecules that behave differently inside a living thing than outside.
Put the point the other way around (and more in the spirit of the FTA), and ask what would we expect to see under the theistic hypothesis? Other things being equal, we would expect to see lots of conundrums and lots of special-exception type laws that allow life to exist, because there are many more ways to make a life-containing universe that include those than there are ways that exclude them. The fact that we don't see such laws and conundrums, then, is evidence against the theistic hypothesis.