“Is There A Buddha?” - The Milindapanha
The king said:
“Have you, Venerable Nāgasena, seen the Buddha?”
'No, Sire.”
Then have your teachers seen the Buddha?”
“No, Sire”
“Then, Venerable Nāgasena, there is not a Buddha.”
“But, great king, have you seen the river Uha in the Himalaya mountains?”
“No, Sir.”
“Or has your father seen it?”
“No, Sir.”
“Then, your Majesty, is there therefore no such river?”
“It is there. Though neither I nor my father has seen it, it is nevertheless there.”
“Just so, great king, though neither I nor my teachers have seen the Blessed One, nevertheless there was such a person.”
“It is very reasonable, Venerable Nāgasena!”
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The Milindapanha: The Questions of King Milinda. Book 3, Chapter 5, Part 1: Is there a Buddha?