When No News is Good News:

Now, it might be thought an amazing coincidence if Earth were the only planet in the galaxy on which intelligent life evolved. If it happened here, the one planet we have studied closely, surely one would expect it to have happened on a lot of other planets in the galaxy–planets we have not yet had the chance to examine. This objection, however, rests on a fallacy: it overlooks what is known as an “observation selection effect.” Whether intelligent life is common or rare, every observer is guaranteed to originate from a place where intelligent life did, in fact, arise. Since only the successes give rise to observers who can wonder about their existence, it would be a mistake to regard our planet as a randomly selected sample from all planets. (It would be closer to the mark to regard our planet as a random sample from the subset of planets that did engender intelligent life, this being a crude formulation of one of the saner ideas extractable from the motley ore referred to as the “anthropic principle.”)