clipped from www.prospect-magazine.co.uk

No self-respecting theist would go so far as to claim that "you cannot prove the non-existence of God" entails "God exists." As mentioned, their point is merely to leave open the possibility that such a being might exist. But Sagan's dragon dashes even this hope. For one can show that it is absurd, irrational, intellectually irresponsible or even lunatic to believe that fairies, goblins, the Norse gods, the Hindu gods, the gods of early Judaism (yes, there were several: go check), and so endlessly on, "might exist." It would compound the felony a millionfold to grant this and yet insist that one's own (Christian or Muslim, say) deity "nevertheless" exists or might exist.

For a simple case of proving a negative, by the way, consider how you prove the absence of pennies in a piggy-bank.