The anthropic principle lets you compute the posterior probability of some value V of the world, given an observable W. The observable can be the number of humans who have lived so far, and the value V can be the number of humans who will ever live. The probability of a V where 100W < V is smaller than the probability of a V only a few times larger than W.
This argument could have been made by any intelligent being, at any point in history, and up to 1500AD or so we have strong evidence that it was wrong every time. If this is the main use of the anthropic argument, then I think we have to conclude that the anthropic argument is wrong and useless.
I would be interested in hearing examples of applications of the anthropic argument which are not vulnerable to the "depending on your reference class you get results that are either completely bogus or, in the best case, unverifiable" counterargument.
(I don't mean to pick on you specifically; lots of commentors seem to have made the above claim, and yours was simply the most well-explained.)
This argument could have been made by any intelligent being, at any point in history, and up to 1500AD or so we have strong evidence that it was wrong every time. If this is the main use of the anthropic argument, then I think we have to conclude that the anthropic argument is wrong and useless.
First, "the anthropic argument" usually refers to the argument that the universe has physical constants and other initial conditions favorable to life, because if it didn't, we wouldn't be here arguing about it.
Second, what you say is true, but someone ...
...at least not if you accept a certain line of anthropic argument.
Thomas Nagel famously challenged the philosophical world to come to terms with qualia in his essay "What is it Like to Be a Bat?". Bats, with sensory systems so completely different from those of humans, must have exotic bat qualia that we could never imagine. Even if we deduce all the physical principles behind echolocation, even if we could specify the movement of every atom in a bat's senses and nervous system that represents its knowledge of where an echolocated insect is, we still have no idea what it's like to feel a subjective echolocation quale.
Anthropic reasoning is the idea that you can reason conditioning on your own existence. For example, the Doomsday Argument says that you would be more likely to exist in the present day if the overall number of future humans was medium-sized instead of humongous, therefore since you exist in the present day, there must be only a medium-sized number of future humans, and the apocalypse must be nigh, for values of nigh equal to "within a few hundred years or so".
The Buddhists have a parable to motivate young seekers after enlightenment. They say - there are zillions upon zillions of insects, trillions upon trillions of lesser animals, and only a relative handful of human beings. For a reincarnating soul to be born as a human being, then, is a rare and precious gift, and an opportunity that should be seized with great enthusiasm, as it will be endless eons before it comes around again.
Whatever one thinks of reincarnation, the parable raises an interesting point. Considering the vast number of non-human animals compared to humans, the probability of being a human is vanishingly low. Therefore, chances are that if I could be an animal, I would be. This makes a strong anthropic argument that it is impossible for me to be an animal.
The phrase "for me to be an animal" may sound nonsensical, but "why am I me, rather than an animal?" is not obviously sillier than "why am I me, rather than a person from the far future?". If the doomsday argument is sufficient to prove that some catastrophe is preventing me from being one of a trillion spacefaring citizens of the colonized galaxy, this argument hints that something is preventing me from being one of a trillion bats or birds or insects.
And this could be that animals lack subjective experience. This would explain quite nicely why I'm not an animal: because you can't be an animal, any more than you can be a toaster. So Thomas Nagel can stop worrying about what it's like to be a bat, and the rest of us can eat veal and foie gras guilt-free.
But before we break out the dolphin sausages - this is a pretty weird conclusion. It suggests there's a qualitative and discontinuous difference between the nervous system of other beings and our own, not just in what capacities they have but in the way they cause experience. It should make dualists a little bit happier and materialists a little bit more confused (though it's far from knockout proof of either).
The most significant objection I can think of is that it is significant not that we are beings with experiences, but that we know we are beings with experiences and can self-identify as conscious - a distinction that applies only to humans and maybe to some species like apes and dolphins who are rare enough not to throw off the numbers. But why can't we use the reference class of conscious beings if we want to? One might as well consider it significant only that we are beings who make anthropic arguments, and imagine there will be no Doomsday but that anthropic reasoning will fall out of favor in a few decades.
But I still don't fully accept this argument, and I'd be pretty happy if someone could find a more substantial flaw in it.