The Anthropic Principle is one of the most controversial conjectures of modern cosmology. And it's not just controversial among philosophers and scientists, but among Forteans as well... as evidenced by the less than warm reception given to the talk by Lynn Picknett and Clive Prince at last weekend's Fortean Times Unconvention. I've always been a big admirer of Lynn and Clive's historical researches, but I have to admit that when they strayed onto scientific subjects they rapidly got out of their depth (I'm sure I get equally out of my depth when I stray onto historical subjects... which I will do, further down this post!).
Current cosmological models involve a number of parameters that are observed to have certain values, but the theory doesn't explain why they have these values and not other ones. Yet if any of the parameters differed by just a small fraction from their actual value, human beings would never have existed to ask the question. Therefore even if the a priori probability of the universe is very small, the fact that human beings exist makes it an absolute certainty in an a posteriori sense. As Sherlock Holmes famously said: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth".
It occurred to me that the Anthropic Principle extends to other "low probability" events besides the small matter of the creation of the Universe. Another example is the emergence of intelligent life on Earth. Many people (including hardened UFO skeptics) believe that intelligent life is an extremely rare phenomenon in the universe, and that the a priori probability of it emerging on any given planet is extremely low. And they may be right. All we know, from the fact that we exist, is that the probability is non-zero!
As I alluded to earlier, you can even extend the Anthropic Principle to historical speculation. Throughout recorded history there have been branch points at which humanity might have been wiped out, or at the very least civilized society might have crumbled... but it didn't. And of course it didn't, or I wouldn't be talking to you now via the World Wide Web. But that doesn't mean that in 999 out of a thousand alternate time-lines human history didn't come to an abrupt end.
A good example is the Third World War. Throughout the 1950s and 60s, there was a widespread belief in the inevitability of a nuclear confrontation between the Soviet Union and the West that would have resulted in the decimation of the world's population and the destruction of civilization as we know it. Countless science fiction novels and short stories of that period either portrayed such a war, or the devastated world that would be all that remained after it. Most of those stories were set in what is now the past -- and yet here we are, still blissfully unobliterated. Maybe there was a 99.99% chance of a world-ending nuclear conflict in the 1960s, but in retrospect 0.01% is as good as 100%. And maybe other low probability events can be explained the same way. What about the physics-defying trajectory of the bullet that killed JFK? Highly improbable, certainly -- but maybe he was the one who started World War Three in those 99.99% of alternative time-lines!