Until quite recently we knew of only one life-harbouring planet in a single planetary system — adrift within a universe of more than a billion trillion stars. Our home was that single speck, the lone bulb in a great cosmic garden, and it raised essentially the same question: is this all? Or are there more?
We might imagine that our very existence in this vast universe makes the existence of other life a foregone conclusion, or at least very likely. But that conceit is profoundly misleading; it’s a victim of one of the most challenging aspects of statistical inference and probability. It’s an example of post-hoc analysis or a posteriori probability — that is, the evaluation of the significance of events that have already happened. This is a treacherous terrain, a place where statisticians know to tread carefully, because rare and common events are indistinguishable once they’ve occurred. And this caution is especially important for those phenomena for which we have few or no precedents. Just because life did arise on earth says nothing, in itself, about how likely it is to arise elsewhere.