As it happens, this version of the cosmological argument owes more to Islamic than to Christian thought. Even though the Catholic Church made it an article of faith (at the Fourth Lateran Council in 1215) that the world had a beginning in time, Christian philosophers such as Thomas Aquinas thought that this could never be demonstrated philosophically. (Thus Aquinas tried to prove God's existence from the weaker assumption that the world might be eternal.) But in the kalam tradition--the Islamic counterpart of the Talmud, so to speak--thinkers like al-Ghazali (1058-1111) sought to show that the very idea of an infinite past was absurd. From this they deduced that the world must have been brought into being at a definite moment by a Creator.

Is the idea of an infinite past really absurd? It is certainly paradoxical. Wittgenstein imagined coming across a man saying to himself, "9 ... 5 ... 1 ... 4 ... 1 ... 3 ... finished." Finished what? you ask him. "Oh, I've been reciting all the digits of pi backward from eternity, and I finally got to the end!" But thanks to the big bang, the temporal finitude of the world does not have to be proved a priori, as the Islamic philosophers attempted to do, or taken on the authority of the Vatican. It is an empirical fact.