If the basic laws of physics were even slightly different, intelligent life would be impossible. For example, if gravity were slightly weaker then all stars would be blue giants and a life permitting star like our own would be impossible. But if gravity were slightly stronger then all stars would be red dwarfs and life would be impossible. This is one of many improbably coincidences that seem to lead to the conclusion that life is only possible because of the work of an intelligent designer. 'God and Design' is a balanced book that includes essays from top scientists and philosophers from both sides of the debate. I would strongly recommend it to anyone who is not interesting in a fair hearing from both sides rather than "preaching to the choir."
Eliot Sober has a strong article defending the weak anthropic principle, which holds that we are only capable of observing a universe that is compatible with our existence. The principle is correct as far as it goes, but it is too limited to explain our surprise at the improbable nature of a life permitting universe. The philosopher John Leslie has a clever thought experiment that illustrates the weakness of the weak anthropic principle. Imagine you faced a firing squad of 100 men and you lived through it unharmed. You wouldn't invoke the weak anthropic principle and claim that they all must have accidentally missed, you would instead suspect that there is some deliberate plan (design). This has led most atheists to posit the existence of billions upon billions of alternate universes - see Martin Rees' essay for a representative example.
Elliot Sober does not give up so easily. He defends the weak anthropic principle on the basis of likelihood analysis, rather than the standard Bayesian analysis preferred by most philosophers. The likelihood principle only focuses on the probability of getting a certain result - in this case a life permitting universe - given that a theory is true. When paired with the weak anthropic principle the likelihood of a life permitting universe is just as good with chance as with design. The problem with likelihood analysis is that it seems to lead to absurd conclusions, such as the prisoner deciding that the firing squad accidently missed. This seems unreasonable, and indeed Sober backs off from that conclusion. In the case of the firing squad we do have the ability to assign meaningful prior probabilities - firing squads are a really good way to kill someone. But Sober claims that we can't make any reasonable claim about the prior probability of the universe supporting life. In this way, Sober parts company from other atheists such as Martin Rees, who do feel a need to directly grapple with and ultimately explain the apparent fine tuning. If Darwin had felt the same way as Sober, he would never have needed to find an explanation for the origin of species. He could have fallen back on the likelihood principle and chance.
Martin Rees was impressed by the firing squad analogy and championed scientific explanations for the apparent fine tuning of the universe. They all involved the multiple universe theory. For example, the physicist Lee Smolin created a cosmic version of Darwin's evolutionary theory. Suppose new universes formed at the "other end" of black holes, and suppose that the physical constants can take on slightly altered values each time a new universe is formed. Then Smolin's theory predicts that those universes most suited to intelligent life are also most suited to forming black holes. However, in the debate book God?: A Debate between a Christian and an Atheist (Point/Counterpoint Series (Oxford, England).) William Lane Craig cites a recent physics study showing that universes most suited towards forming black holes would actually prohibit star formation - bodies that massive would go directly to black holes. The physicist Leonard Susskind points out that we now know that black holes do not lose information, but Smolin's theory requires that some information from our universe is lost. Finally the physicist Alexander Vilenkin has shown that a universe with a higher cosmological constant would actually maximize black holes. These are still open issues and the responses and counter-responses are flying, but so far it does not look good for Smolin's theory. And as Smolin points out, his theory is the only multiverse theory to make testable predictions.
Ironically, what was perhaps the most powerful challenge came from the Christian philosophers Tim and Lydia McGrew along with Eric Vestrup. If you entered a lottery with a million tickets then your chances of winning are one in a million. But what is your chance of winning if there are an infinite number of tickets? Similarly, the physical constants have a (presumably) infinite range. So it becomes problematic to say that there is a low probability of a life permitting universe. This is called the renormalization objection. Of course, the larger the range of possible values for the physical constants, the lower the probability. And the limit is zero. This should potentially kill chance as an explanation. But here the McGrew's raise the coarse tuning argument. We are surprised to find that the universe is fine tuned within a range of a few percent. But we would not be surprised if the universe only required "coarse tuning" of, perhaps, 40 or 50 orders of magnitude. However, as the upper range for the constants approaches infinity, the probability of even coarse tuning goes to zero. The purported challenge for design advocates is to defend fine tuning while rejecting coarse tuning.
However, this challenge is unconvincing. If the constants really could range up to infinity, then a mere 40 or 50 orders of magnitude is just a drop in the bucket. The reason why coarse tuning seems unsurprising is because we tacitly expect that the upper bound for the constants is about 50 or so orders of magnitude above zero. We don't really expect that the nuclear strong force could be as high as a googolplex (or much higher).
The Christian philosopher and mathematician Alexander Pruss has another answer that is sufficient to sweep away any remaining objections to the renormalization problem (he says it convinced the McGrew's). Regrettably, it was not included in the book, but it is available online. Pick an arbitrary boundary, perhaps a googolplex, and separate the range into two pieces. Then there are only two possibilities: the constant is less than the boundary, or it is higher than the boundary. Even if we can't create a suitable outer measure that tells us the probability by chance of being above or below the boundary, it does not matter. If the constant falls into the lower range then the chance of a life permitting universe is extraordinarily improbable. But if the constant falls into the larger range, then the chance is zero. Either way, fine tuning through chance must be rejected.
On the Christian side, William Lane Craig contributed an excellent essay that defends design against multiple objections. But the best article was by the philosopher Roger White. He shows that even the multiple universes argument defended by Martin Rees and others suffers from the inverse gambler's fallacy. The standard gambler's fallacy is that you are more likely to win after a losing streak because you are "due." In reality, the chances of each trial are independent. The inverse gambler's fallacy is that when you start gambling and get a winning streak it was because someone else had just gone through a losing streak, so the game was already "due" for a winner.
The multiple universes explanation suffers from this fallacy. Even if there are countless billions of alternate universes in other parallel dimensions, that does not increase the chance of our universe supporting intelligent life. It only means that one of the countless billions of other universes has some type of intelligent life. The odds of our universe being that lucky winner are extremely improbable. This is good point, but I think that some multiple universes theories avoid this weakness, such as Smolin's theory of black holes. If the "multiverse" is evolving towards supporting life, then ultimately every universe in the countless billions will be life-permitting. I think that is the direction future scientific theories for multiple universes should tend.
There is a lot in the book, and my review has barely scratched the surface of the rebuttals and counter-rebuttals. A great choice if you want to get up to date on the issue.