Comment by AnimalMuppet

This fits the pattern of discussion I often see about the anthropic principle. Someone (I'll label them the "Creationist", though it could just as well be an advocate of intelligent design) asserts that the probability of things "just happening" is vanishingly small. Sometimes the Creationist cites some generally-accepted science, and sometimes does a probability calculation, resulting in a very small probability that things happened by purely naturalistic means.

Someone else (call them the "Evolutionist") responds with the anthropic principle - that, if no intelligent life had arisen in this universe, there would be nobody here to observe that there was no intelligent life. And this is completely logically correct. It is also irrelevant. The Creationist never asserted that it was improbable that life arose in this universe, but rather that it was improbable that it arose purely by naturalistic means. The question isn't whether we're here; the question is how or why.

The Creationist was saying, either we're here by purely naturalistic, evolutionary means, with some probability (call that Pe), or by being created, with some probability (call that Pc). As far as I can see, Pc is unknowable, even in principle. But the Creationist argument is that Pe is so low that it seems reasonable that Pc is higher. That is, it seems reasonable to suppose that we are here due to creation, not just evolution.

The anthropic principle doesn't answer that argument at all. It gives an argument about "whether", not about "how".

Or to put it in different terms: The anthropic principle says something like, if there are a billion universes, and life only arose in a thousand of them, we have to be in one of those thousand to be having this conversation. (Note that I don't actually believe in multiple universes; this is just to make the probability discussion clearer.) But the Creationist never denied that. The Creationist says: Of those thousand universes, if life arose by creation in 998 of them and by evolution in only 2 of them, it seems reasonable to suppose that we're in a universe where life originated by creation, not evolution. The anthropic principle, which asserts that we're in one of the thousand, doesn't address the Creationist's argument at all.

Unless.

It seems to me that everyone who pulls out the anthropic principle in this situation implicitly assumes that Pc is precisely zero. They never explicitly state this assumption, but I think it's there in their thinking. So for the Evolutionist in this conversation, Pe and the probability of life at all are exactly the same, and the anthropic principle does address the actual claim.

But, instead of being irrelevant, in this case the anthropic principle is begging the question. The Evolutionist starts with the conclusion that they are arguing for. That's invalid logic. That's so invalid that, to the degree that the Evolutionist relies on the anthropic principle to support their position, to that degree they should doubt their position.

(I think the Evolutionist pulls out the anthropic principle for an additional reason - it's easy. It lets them "win" the discussion without having to disprove the Creationist's big scary probability number.)