I close my eyes and see a flock of birds. The vision lasts a second or perhaps less; I don’t know how many birds I saw. Were they a definite or an indefinite number? This problem involves the question of the existence of God. If God exists, the number is definite, because how many birds I saw is known to God. If God does not exist, the number is indefinite, because nobody was able to take count. In this case, I saw fewer than ten birds (let’s say) and more than one; but I did not see nine, eight, seven, six, five, four, three, or two birds. I saw a number between ten and one, but not nine, eight, seven, six, five, etc. That number, as a whole number, is inconceivable; ergo, God exists.
[From Dreamtigers, by Jorge Luis Borges, translated by Mildred Boyer]
[This would have made more sense (or less) if all he had written was: "The number, as a whole number, is inconceivable; ergo, God exists." But that wouldn't have been Borges. Apparently, his entire argument hangs on the word "inconceivable" (spoken with a lisp) . . . but how that word makes this a sound argument, I don't know . . . I love Borges.]

I wrote an article on proof God exists that you might find interesting. Here’s the link:
http://jasonbrophy.wordpress.com/2010/05/25/proof-god-exists/
Having read many things written by Borges, I doubt that he intended this as a proof. It is partly a parody of proofs of the existence of God, partly a sincere expression of faith (which does not care whether it is a proof or not), partly a reverie.
I agree with Phil, there’s a sort of sarcasm in this “proof”. Look at the title: is “Argumentum Ornithologicum” serious? However the proof is incorrect. Godel demostrated the existence of arithmetic universes with integer numbers different than 2,3,4,5,6,7,8,9.
This implies that there are undefined numbers other than defined numbers 1,2,3,4,… So whe have to admit that this argument is against the existence of God!
ps: sorry for my not-so-good english!