From a wonderful little book by Ratzinger/Benedict, Christianity and the Crisis of Cultures:

It is an obvious fact that the rational character of the universe cannot be explained rationally on the basis of something irrational!

It is obvious, though I don’t know that it can be proven, and yet there are many who would deny it. I haven’t been able to figure out how to argue with someone who doesn’t see that the problem Ratzinger names is a problem.