Tautologies are not too helpful
It is clear that if it is possible to prove, using valid arguments, that life couldn't exist under conditions XY, then life couldn't exist under conditions XY. But there's always a lot of choices where life could exist and scientists want to understand which of these choices are correct and why.
The anthropic reasoning is not helpful to achieve this goal. If you are into soccer betting and you want to know the result of a match, the anthropic fallacy tells you that the score probably won't be "20070808 : 1" because the soccer players would probably die before this score is reached. Is that helpful? I don't think so. What you really want to know is whether the result will be "3 : 0" or "1 : 2" or another realistic pair of integers. Almost by definition, if a hypothetical result leads to a destruction of life, it isn't a terribly intelligent possibility to be considered in the first place. We have other, more solid methods to figure out that the final score won't be "20070808 : 1": we don't have to exterminate soccer teams to get a realistic idea about the possible results.
For example, if you want to determine the cosmological constant, you can measure the acceleration of the expanding Universe.
The conclusion about the weakness of the anthropic reasoning can be rephrased in another way: if the anthropic reasoning can tell you that XY is impossible, it doesn't mean that it is the only argument or the best argument or the most satisfactory argument to answer such questions. Quite on the contrary. For example, there exists a methodology to answer questions that is called "science". So far, all acceptable answers found by this method were non-anthropic in character. They turned out to be able not only to show that some results had to be incorrect: science was even able to say what results are correct! ;-)
The examples of his "sensible anthropic principle" that Sean offers us are all examples showing that our civilization is located in a special environment that differs from the majority of the Universe or its relevant part. We are made out of baryonic matter even though most of the particulate mass of the Universe is composed of dark matter. We live on a rock even though the mass of the planets is mostly stored in gas giants, and so forth. You can surely invent dozens of such examples and there is no need to read Sean's list.
However, it is very clear that these random observations are not useful to understand anything new about the Universe. Why do I say that his examples are random? Isn't it a general fact that our environment has extreme characteristics that differ from the majority of the Universe? Well, be sure that it is not a general fact. The "sensible anthropic principle" is extremely far from being a universal principle.
This "principle" is simply not true.
Below we will offer other examples showing that our civilization lives in one of the most average and generic types of environments that are possible. It is kind of interesting that this principle - the goldilocks principle - is being frequently used by the champions of the anthropic reasoning, too. The proponents of the anthropic reasoning want us to be impressed whenever our environment differs from a majority of the Universe; the same proponents want us to be impressed whenever our environment turns out to be typical, too.
And there exists a huge number of these examples, too:
- the Solar System is located neither at the center of Milky Way, nor near its border: it is somewhere in between;
- the Sun is neither one of the heaviest stars nor one of the lightest ones: the Sun is an average star;
- the Earth is neither the heaviest, nor the lightest planet of the Solar System but rather an average one;
- the distance of the Earth from the Sun is neither too small, nor too high;
- the temperature on Earth is neither too low nor too high; compare it with Venus or Mars;
- the essential element behind life - carbon - is neither one of the lightest elements nor one of the heaviest elements;
- Sean Carroll is the citizen of the economically strongest country in the world.
You can surely invent dozens if not hundreds of other examples from all kinds of science, too. When we combine all these insights, the punch line is that sometimes our environment is special, unusual, and extreme; sometimes it is typical, generic, and average. There exists no universal answer to the question whether our environment is special in some respect or not: the correct answer always depends on the respect.
Everyone who thinks that the answer to this question is universal - and can thus be used as a law of physics - is cheating himself and his list of entries supporting such a position is nothing else than a deliberately biased random list of cherry-picked arguments. Interestingly enough, the proponents of the anthropic fallacy exhibit both kinds of biases. Sometimes they argue that our environment has to be very special ("sensible anthropic principle") and everyone should view this statement as a gospel; sometimes they argue that our environment has to be very generic and ordinary ("the Copernican or mediocrity principle") and everyone should accept this opinion as a gospel, too.
Both of these viewpoints are irrational biases. No one seems to care that these two biases directly contradict one another. Incidentally, there exists a meta-bias that determines which of these two approaches is emphasized in a given context. Whenever the alternative environments - such as other planets - have already been observed, the anthropic people choose to emphasize the "sensible anthropic principle" and argue that our environment looks very special. Why do they do so? Because they want to convince others that the "principle" is able to correctly answer non-trivial questions. However, whenever the alternative environments are unknown - such as other universes in the multiverse - the anthropic people choose to believe that our environment has to be a generic one: that is the reason, for example, why some people calculate "statistics" in the landscape.
Their whole reasoning is not only disconnected from the actual physical arguments that decide about particular answers; it is also logically inconsistent.
Once again, some quantities describing our environment or our Universe are rather extreme in comparison with conceivable alternatives; others are generic. This is also true in the context of questions that haven't yet been answered: some of them will have special answers, others will have generic answers. The right answer depends on the question we ask and can only be found with the help of detailed, rational arguments, not with the help of anthropic dogmas and biases. Everyone who thinks that the answer doesn't depend on the question is a victim of a thoroughly irrational religion.
And that's the memo.