In part 1 I explained what the Anthropic Principle is and gave some examples to illustrate it. Basically, the idea is that certain conditions must be met in order for life to be possible anywhere in the universe at any time in its history. These conditions are so numerous and so unlikely that, when added together, they make it virtually impossible for there to be a planet capable of supporting life -- at least advanced life -- anywhere in the universe if left to chance. Since there is a planet capable of supporting life (Earth in case you were wondering) it suggests that it wasn't left to chance, that someone intended that the universe would be hospitable to life.
However, while the basic facts are not disputed by scientists in the relevant disciplines, the theistic conclusion is a matter of controversy; some scientists accept it, others don't (here's an interesting sample). In this post I'll go over some of the most common objections made against this inference. I'm saving two objections that require lengthier responses for the next post.
1. "We would not be here to observe the universe unless the very unlikely did happen, so of course we're going to notice how the universe 'just happens' to have the necessary conditions for life." This was my first thought when I heard about the Anthropic Principle for the first time. However, it really doesn't hold any water if you think about it for more than a few seconds. The fact that we are here of course proves that the necessary conditions for life's existence have been met, regardless of how unlikely it is; but the question is not whether these conditions have been met but how they've been met. And the fact that they are unlikely to the point of being impossible shows that they were not met by chance.
The common analogy I've seen in the philosophical literature is the firing squad. If a man were sent to be executed by a hundred sharpshooters and he survives the experience, he could draw two conclusions: they all missed by chance, or they intended him to live (by missing on purpose or filling the guns with blanks for instance). But he would not take the fact that he was alive as evidence that it happened by chance. He would not say, "I wouldn't be here to observe the fact that I'm alive unless I survived -- therefore they must have missed accidentally." In fact the more unlikely his "being alive" was, the more rational it would be for him to conclude that someone decided he should live. In the same way the rational conclusion to draw from the incredible degree of fine-tuning we find in the universe is that someone decided we should live.
2. "We don't have enough information to warrant drawing any conclusions, much less theistic ones." There is certainly some truth to this; the Anthropic Principle is a relatively young field of study, and we should bear this in mind. However, it should also be borne in mind that all of the research has consistently pointed in the same direction: that the prerequisites for life's existence are very specific. Perhaps future scientific discoveries will overturn this evidence, but this could be said of virtually any scientific claim (although it's less implausible for younger fields of study than older ones). At any rate, this isn't really an objection to the theistic conclusion, but to the data itself, and virtually all scientists in the relevant disciplines acknowledge the data.
I also have to say I find it interesting that when scientific discoveries can be seen as going against belief in God, this objection is rarely given. It's only when science seems to point to God that people start suggesting that we can't really draw any conclusions.
3. "We don't have any other universes to compare this one with, so we can't say how likely or unlikely it is for these conditions to be what they are." In the first post I stated that there are two levels to the Anthropic Principle: the conditions that have to be met within the universe, and the conditions that have to be met in the universe as a whole. The conditions that fall into the latter category are initial conditions that are simply given in the Big Bang itself. Since they are initial conditions, there are no prior physical conditions that force them to be the way they are, by definition. There are only two possible conclusions from this: first, that these initial conditions could have been different. Or second, that there were non-physical conditions forcing the universe's physical conditions to be what they are. The first leads to the problem the Anthropic Principle poses: that the universe simply shouldn't be able to support life if left to its own resources, and yet it does. The second leads to the conclusion that there is some non-physical reality, external to the universe, that is able to exert some degree of power over the universe. Thus, both of these conclusions have theistic repercussions.
As for the the conditions that have to be met within the universe, some of them are indeed speculative. For example, while we have no reason to think that planets inherently form with exactly the same surface gravity or axial tilt as Earth, we have not discovered enough extra-solar planets to test it directly. However, many of these conditions are not speculative, but are easily calculable. For example, in order for a planet to be able to support life it must be in a certain type of galaxy, in a certain part of the galaxy, orbiting a certain type of star, etc. It is easily observable and demonstrable how common these conditions are.
Moreover, the fact that we have a sample size of one actually supports the theistic conclusion. This will be demonstrated in the response to one of the objections in the next post.
4. "Chance and intent are not the only two explanations possible. There's also natural law. If there's a law which makes the universe and planets capable of supporting life, the odds of there being other possible life-sites in the universe would be very likely." Well, as pointed out above, the conditions necessary for the universe as a whole are initial conditions. As such, there is no preceding natural law forcing them to be they way they are by definition. The necessary conditions within the universe could, theoretically, be shown to be the result of as-yet-undiscovered natural laws. But in the absence of any evidence for such laws, this suggestion is completely ad hoc, since virtually anything could be explained as the result of some natural law we just haven't discovered yet. Besides, this would only push the problem back to the level of the universe as a whole: any law that makes the universe hospitable to life would have to be exactly what it is in order to ensure that the specific properties necessary for the existence of life are met. The universe would still be fine-tuned for the existence of life, and we'd still need an explanation for why this is the case.
5. "If you change one physical constant it may throw everything off-kilter, but then you can change the other physical constants to compensate for it, and bring it back to being a universe hospitable to life." Incredibly enough, scientists already thought of that. The obvious problem is that changing the other constants does not merely compensate for changing the first one; it would also have dramatic effects which would require us to change more constants, which would have their own effects requiring further changes, etc. There are a few cases where you could do this and end up with a life-permitting universe, but they would be extremely rare. It's like taking a medication that has significant side effects. You then have to take other medications to regulate these side effects, but then these medications also have side effects, so you need to take more medications...
Of course, this analogy only goes so far: taking more medications may actually bring some degree of health to the body. With the universe's physical constants, however, it is almost impossible to alter them and still end up with a life-permitting universe.
6. "Someone wins the lottery, and it would be irrational for that person to think that the extreme improbabilities involved in her winning would demonstrate that someone set it up for her to win. Similarly, life is the result of this universe. This doesn't allow us to think it was set up intentionally to be this way." To illustrate this objection, say you had, for some ungodly reason, billions of ping pong balls and inscribed each one with someone's name until you had one for every person in the world. Also say you had a big enough basket to hold all of them. You then mix them all up and pull one out while blindfolded. Obviously someone's name will be drawn, even though the odds were one in several billion that you would select that particular ball. Similarly, the fact that we have a universe with the specific properties it has may have been improbable, but that does not allow us to draw any conclusions about whether it was "arranged."
But this is a bad analogy. A better one would be if, every time you tweak the universe's properties, you paint a ping pong ball black if it permits the existence of life, and just leave it white if it does not. What you would end up with is an ocean of white ping pong balls with only a handful of black ones scattered throughout. Now of course the odds that you would pull any particular ping pong ball out is equally improbable; but the odds that you would pull out a white ping pong ball is enormously more probable than the odds that you would pull out a black one. Similarly, the odds that the universe would be life-prohibiting is vastly more probable than for it to be life-permitting -- unless someone decided to make it hospitable to life.
This objection seems to be suggesting that life -- the result of this universe -- is arbitrary. Any other universe would have had results too. The problem with this is that all the other possible universes would have had essentially the same result: just matter and energy in motion, and often not even the motion. Ours has something on a whole different level, and the universe must be balanced on a razor's edge in order for it to be that way.
Update (13 Feb): See also part 1, part 3, and part 4.
(cross-posted at Quodlibeta)