Why does the universe exist at all? Several writers, including most notably John Leslie and more recent Derek Parfit, have argued that the answer “It just does” is not one that we should willingly embrace. For the argument goes, we know both that this universe contains life, and that the probability of its containing life is, on the face of it, extremely low. It seems that the conditions obtaining at the moment of the universe's coming into being could very easily have been such as to render life impossible. Thus the universe appears to have been “fine-tuned” for life.

How, then, is this fine-tuning to be explained? Two suggestions need to be considered. One is that the universe is, after all, created by God, who purposely fashioned its initial conditions such that life would evolve. The other is that this is in fact one among a very large number of actual universes, most of which are incompatible with life's development. Given this large number, and the randomness of their initial conditions, it is now much less surprising that some small portion will be suited to life. It is unsurprising too that we should find ourselves in such a universe, rather than elsewhere.

Leslie offers a number of analogies that help to illustrate these claims. Imagine coming across a solitary monkey who has just sat down and typed out the words to the Battle Hymn of the Republic.