"I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
"But," says Man, "the Babel fish is a dead giveaway isn't it? It could not have evolved by chance. It proves that you exist, and so therefore, by your own arguments, you don't. Q.E.D. "
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic*
*La Guia del Viajero Intergalactico (Douglas Noël Adams)