Raymond Smullyan – The Tao Is Silent (1977)
MORTAL: All right, I’ll grant your point! But what I really want to know is do you exist?
GOD: What a strange question!
MORTAL: Why? Men have been asking it for countless millennia.
GOD: I know that! The question itself is not strange; what I mean is that it is a most strange question to ask of me!
GOD: Because I am the very one whose existence you doubt! I perfectly well understand your anxiety. You are worried that your present experience with me is a mere hallucination. But how can you possibly expect to obtain reliable information from a being about his very existence when you suspect the nonexistence of the very same being?
excerpt (“Is God a Taoist?”) appears in The Mind’s I, ed. Daniel Dennett and Douglas R. Hofstadter, 1981