Can you explain to me this quote, from Terry Pratchett?

" In the second scroll of Wen the Eternally Surprised a story is written concerning one day when the apprentice Clodpool, in a rebellious mood, approached Wen and spake thusly: "Master, what is the difference between a humanistic, monastic system of belief in which wisdom is sought by means of an apparently nonsensical system of questions and answers, and a lot of mystic gibberish made up on the spur of the moment?" Wen considered this for some time, and at last said: "A fish!" And Clodpool went away, satisfied. "